Unum (UNM) 10-Q 2 May 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	Apr. 30, 2018
Document Document and Entity Information [Abstract]
Entity Registrant Name	Unum Group
Trading Symbol	UNM
Entity Central Index Key	5,513
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Common Stock, Shares Outstanding		221,194,578

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investments
Fixed Maturity Securities - at fair value (amortized cost: $40,151.9; $39,780.5)	$ 44,500.1	$ 45,457.8
Mortgage Loans	2,200.8	2,213.2
Policy Loans	3,573.3	3,571.1
Other Long-term Investments	706.5	646.8
Short-term Investments	712.6	1,155.1
Total Investments	51,693.3	53,044
Other Assets
Cash and Bank Deposits	308.9	77.4
Accounts and Premiums Receivable	1,736.7	1,665.7
Reinsurance Recoverable	4,815.5	4,879.2
Accrued Investment Income	745.8	690.1
Deferred Acquisition Costs	2,213.4	2,184.6
Goodwill	348.9	338.6
Property and Equipment	508.4	504.8
Other Assets	641.4	628.7
Total Assets	63,012.3	64,013.1
Liabilities
Policy and Contract Benefits	1,623.1	1,605.2
Reserves for Future Policy and Contract Benefits	44,668.5	45,601.6
Unearned Premiums	432.8	373.1
Other Policyholders' Funds	1,635.6	1,595
Income Tax Payable	61.3	2.9
Deferred Income Tax	128.9	199
Short-term Debt	200	199.9
Long-term Debt	2,721.9	2,738.4
Payables for Collateral on Investments	382.2	396.2
Other Liabilities	1,658.5	1,726.9
Total Liabilities	53,512.8	54,438.2
Commitments and Contingent Liabilities - Note 11
Stockholders' Equity
Common Stock, $0.10 par; authorized: 725,000,000 shares; issued: 304,951,984 and 304,448,032 shares	30.5	30.5
Additional Paid-in Capital	2,302.4	2,303.3
Accumulated Other Comprehensive Income (Loss)	(82.4)	127.5
Retained Earnings	9,777.8	9,542.2
Treasury Stock - at cost: 83,783,327 and 81,900,950 shares	(2,528.8)	(2,428.6)
Total Stockholders' Equity	9,499.5	9,574.9
Total Liabilities and Stockholders' Equity	$ 63,012.3	$ 64,013.1

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investments
Fixed Maturity Securities, Amortized Cost	$ 40,151.9	$ 39,780.5
Stockholders' Equity
Common Stock, Par	$ 0.10	$ 0.10
Common Stock, Authorized	725,000,000	725,000,000
Common Stock, Issued	304,951,984	304,448,032
Treasury Stock, Shares at Cost	83,783,327	81,900,950

CONSOLIDATED STATEMENTS OF INCO

CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue
Premium Income	$ 2,250	$ 2,142.9
Net Investment Income	602.3	602.4
Realized Investment Gain (Loss)
Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	(1)	0
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	(1.2)	11
Net Realized Investment Gain (Loss)	(2.2)	11
Other Income	49.5	50.2
Total Revenue	2,899.6	2,806.5
Benefits and Expenses
Benefits and Change in Reserves for Future Benefits	1,807.9	1,749
Commissions	282.3	270.2
Interest and Debt Expense	40.2	39.8
Deferral of Acquisition Costs	(169.3)	(162.1)
Amortization of Deferred Acquisition Costs	151.5	141.5
Compensation Expense	221.7	209
Other Expenses	224.2	228.8
Total Benefits and Expenses	2,558.5	2,476.2
Income Before Income Tax	341.1	330.3
Income Tax (Benefit)
Current	89.4	56.6
Deferred	(21.8)	43.8
Total Income Tax	67.6	100.4
Net Income	$ 273.5	$ 229.9
Net Income Per Common Share
Basic	$ 1.23	$ 1
Assuming Dilution	$ 1.23	$ 1

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net Income	$ 273.5	$ 229.9
Other Comprehensive Income (Loss)
Change in Net Unrealized Gain on Securities Before Adjustment (net of tax expense (benefit) of $(274.9); $89.0)	(1,042.1)	176.9
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance (net of tax expense (benefit) of $212.1; $(67.3))	807.9	(134.5)
Change in Net Gain on Hedges (net of tax benefit of $2.3; $6.0)	(8.8)	(10.7)
Change in Foreign Currency Translation Adjustment	47.5	17.1
Change in Unrecognized Pension and Postretirement Benefit Costs (net of tax expense of $0.7; $1.5)	3.1	2.6
Total Other Comprehensive Income (Loss)	(192.4)	51.4
Comprehensive Income	$ 81.1	$ 281.3

CONSOLIDATED STATEMENTS OF COM6

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Change in Net Unrealized Gain on Securities Before Adjustment, Tax Expense (Benefit)	$ (274.9)	$ 89
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance, Tax Expense (Benefit)	212.1	(67.3)
Change in Net Gain on Hedges, Tax Benefit	2.3	6
Change in Unrecognized Pension and Postretirement Benefit Costs, Tax Expense	$ 0.7	$ 1.5

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock
Increase (Decrease) in Stockholders' Equity
Cumulative Effect of New Accounting Principle in Period of Adoption				$ 0	$ 0
Balance at Beginning of Period, as Adjusted				(51)	8,744
Balance at Beginning of Period at Dec. 31, 2016		$ 30.4	$ 2,272.8	(51)	8,744	$ (2,028.2)
Increase (Decrease) in Stockholders' Equity
Common Stock Activity			4.6
Other Comprehensive Income (Loss)	$ 51.4			51.4
Net Income	229.9				229.9
Dividends to Stockholders (per common share: $0.23; $0.20)					(46.5)
Purchases of Treasury Stock	(100)					(100)
Balance at End of Period at Mar. 31, 2017	9,107.4	30.4	2,277.4	0.4	8,927.4	(2,128.2)
Increase (Decrease) in Stockholders' Equity
Cumulative Effect of New Accounting Principle in Period of Adoption				(17.5)	14.5
Balance at Beginning of Period, as Adjusted				110	9,556.7
Balance at Beginning of Period at Dec. 31, 2017		30.5	2,303.3	127.5	9,542.2	(2,428.6)
Increase (Decrease) in Stockholders' Equity
Adjustment to Additional Paid in Capital, Income Tax Effect from Share-based Compensation, Net			(0.9)
Other Comprehensive Income (Loss)	(192.4)			(192.4)
Net Income	273.5				273.5
Dividends to Stockholders (per common share: $0.23; $0.20)					(52.4)
Purchases of Treasury Stock	(100.2)					(100.2)
Balance at End of Period at Mar. 31, 2018	$ 9,499.5	$ 30.5	$ 2,302.4	$ (82.4)	$ 9,777.8	$ (2,528.8)

CONSOLIDATED STATEMENTS OF STO8

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) (Parenthetical) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Retained Earnings
Dividends to Stockholders, per common share	$ 0.23	$ 0.200

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash Flows from Operating Activities
Net Income	$ 273.5	$ 229.9
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities
Change in Receivables	(29.4)	(18.2)
Change in Deferred Acquisition Costs	(17.8)	(20.6)
Change in Insurance Reserves and Liabilities	183.8	128.7
Change in Income Taxes	56.4	92.5
Change in Other Accrued Liabilities	(110.4)	(18.3)
Non-cash Components of Net Investment Income	(95.2)	(103.7)
Net Realized Investment (Gain) Loss	2.2	(11)
Depreciation	24.6	26.5
Other, Net	12.2	4.4
Net Cash Provided by Operating Activities	299.9	310.2
Cash Flows from Investing Activities
Proceeds from Sales of Fixed Maturity Securities	84.8	154.3
Proceeds from Maturities of Fixed Maturity Securities	469.8	568.3
Proceeds from Sales and Maturities of Other Investments	82.3	46.5
Purchases of Fixed Maturity Securities	(835.4)	(774.7)
Purchases of Other Investments	(90)	(95.7)
Net Sales of Short-term Investments	450.9	10.4
Net Decrease in Payables for Collateral on Investments	(14)	(8.4)
Net Purchases of Property and Equipment	(29.1)	(27.3)
Net Cash Provided (Used) by Investing Activities	119.3	(126.6)
Cash Flows from Financing Activities
Long-term Debt Repayments	(15)	(15.2)
Issuance of Common Stock	1.3	1
Repurchase of Common Stock	(105.7)	(106.9)
Dividends Paid to Stockholders	(52.4)	(46.5)
Other, Net	(15.9)	(9.4)
Net Cash Used by Financing Activities	(187.7)	(177)
Net Increase in Cash and Bank Deposits	231.5	6.6
Cash and Bank Deposits at Beginning of Year	77.4	100.4
Cash and Bank Deposits at End of Period	$ 308.9	$ 107

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	The accompanying consolidated financial statements of Unum Group and its subsidiaries (the Company) have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. For further information, refer to the consolidated financial statements and footnotes included in our annual report on Form 10-K for the year ended December 31, 2017 . In our opinion, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation have been included. Interim results are not necessarily indicative of full year performance.

Accounting Developments

Accounting Developments	3 Months Ended
	Mar. 31, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Accounting Developments	Accounting Developments Accounting Updates Adopted in 2018: Accounting Standards Codification (ASC) Description Date of Adoption Effect on Financial Statements ASC 230 "Statement of Cash Flows" This update provided clarifying guidance intended to reduce the diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. The update addressed eight specific cash flow issues that relate to various types of transactions. The guidance is to be applied retrospectively. January 1, 2018 The adoption of this update resulted in the reclassification of certain cash inflows from investing activities to cash inflows from operating activities within our consolidated statements of cash flows. This reclassification related to cash distributions from equity method investees and the bifurcation of those distributions as either returns on investment or returns of investment. The adoption of this update had no effect on our financial position or results of operations. See the summary table below for the financial statement impacts of this retrospective adoption. ASC 606 "Revenue from Contracts with Customers" These updates superseded virtually all existing guidance regarding the recognition of revenue from customers. Specifically excluded from the scope of these updates are insurance contracts, although our fee-based service products are included within the scope. Our fee-based service products, which are primarily sold in our Unum US segment, are reported in other income within our consolidated statements of income and represent less than one percent of our total revenue. The core principle of this guidance is that revenue recognition should depict the transfer of goods or services to customers in an amount that reflects the consideration to which an entity expects to be entitled in exchange for those goods or services. Accordingly, we continue to recognize revenue for these fee-based service products as services are rendered. The guidance is to be applied retrospectively for all periods presented or retrospectively with a cumulative-effect adjustment at the date of adoption. January 1, 2018 The adoption of these updates did not have an impact on our financial position or results of operations and did not result in expanded disclosures due to the immaterial nature of our fee-based service products relative to our overall business. ASC Description Date of Adoption Effect on Financial Statements ASC 715 "Compensation - Retirement Benefits" This update required the service cost component of net periodic pension and postretirement benefit costs to be included as a component of compensation costs in an entity's statement of income. Other components of net periodic pension and postretirement benefit costs are required to be presented separately from the service cost along with a disclosure identifying the line items in which these costs are presented in the statement of income. The amendments in this update are to be applied retrospectively or prospectively depending on the specific requirement of the update. January 1, 2018 The adoption of this update resulted in the reclassification of service cost from the other expenses line item to the compensation expense line item on our consolidated statements of income but had no effect on our financial position or results of operations. We elected to use the practical expedient for the retrospective application of this update. See the summary table below for the financial statement impacts of this retrospective adoption. ASC 740 "Income Taxes" This update eliminated the exception that required the tax effect of intra-entity asset transfers other than inventory to be deferred until the transferred asset is sold to a third party or otherwise recovered through use. It required recognition of tax expense from the sale of the asset in the seller’s tax jurisdiction when the transfer occurs, even though the pre-tax effects of that transaction are eliminated in consolidation. The guidance is to be applied retrospectively. January 1, 2018 The adoption of this update did not have an impact on our financial position or results of operations. ASC 815 "Derivatives and Hedge Accounting" This update provided targeted improvements to accounting for hedging activities for both nonfinancial and financial risk components, aligns the recognition and presentation of the effects of the hedging instrument and the hedged item in the financial statements, eases certain documentation and effectiveness assessment requirements, and enhances transparency through expanded disclosures. For cash flow and net investment hedges existing at the date of adoption, the guidance is to be applied using a modified retrospective approach through a cumulative-effect adjustment to accumulated other comprehensive income with a corresponding adjustment to retained earnings as of the beginning of the fiscal year the guidance is adopted. The amended presentation and disclosure guidance is required prospectively. Early adoption is permitted. January 1, 2018 We elected to early adopt this update. The adoption of this update did not have an impact on our financial position or results of operations; however, it expanded our disclosures. This update will also simplify hedge documentation requirements and expand available hedging strategies. ASC Description Date of Adoption Effect on Financial Statements ASC 825 "Financial Instruments - Overall" This update changed the accounting and disclosure requirements for certain financial instruments. These changes include a requirement to measure equity investments, other than those that result in consolidation or are accounted for under the equity method, at fair value through net income unless the investment qualifies for certain practicability exceptions. In addition, the update clarified guidance related to the valuation allowance assessment when recognizing deferred tax assets resulting from unrealized losses on available-for-sale fixed maturity securities. Changes also included the modification of certain disclosures around the fair value of financial instruments, including the requirement for separate presentation of financial assets and liabilities by measurement category, as well as the elimination of certain disclosures around methods and significant assumptions used to estimate fair value. The guidance is to be applied using a modified retrospective approach through a cumulative-effect adjustment to accumulated other comprehensive income with a corresponding adjustment to retained earnings as of the beginning of the fiscal year the guidance is adopted. January 1, 2018 See the summary table below for the financial statement impacts of this modified retrospective adoption on our financial statement line items at January 1, 2018. Summary of Financial Statement Impacts of Accounting Updates Adopted in 2018: For the Three Months Ended March 31, 2017 Historical Accounting Method As Adjusted Effect of Change (in millions of dollars) Adjustments due to ASC 230 Consolidated Statements of Cash Flow Cash Flows from Operating Activities Other, Net $ 2.2 $ 4.4 $ 2.2 Cash Flows from Investing Activities Proceeds from Sales and Maturities of Other Investments 48.7 46.5 (2.2 ) Adjustments due to ASC 715 Consolidated Statements of Income Compensation Expense 207.0 209.0 2.0 Other Expenses 230.8 228.8 (2.0 ) Balance at December 31, 2017 Balance at January 1, 2018 Effect of Change (in millions of dollars) Adjustments due to ASC 825 Consolidated Balance Sheets Assets Other Long-term Investments $ 646.8 $ 643.0 $ (3.8 ) Liabilities Deferred Income Tax 199.0 198.2 (0.8 ) Stockholders' Equity Accumulated Other Comprehensive Income (Loss) 127.5 110.0 (17.5 ) Retained Earnings 9,542.2 9,556.7 14.5 For the adoption of these updates, certain reclassifications have been made to prior year amounts in order to conform to current year presentation. Accounting Updates Outstanding: ASC Description Date of Adoption Effect on Financial Statements ASC 220 "Income Statement - Reporting Comprehensive Income" This update allows entities to make an accounting policy election to reclassify the stranded tax effects arising as a result of the recognition of the enactment of the tax bill, H.R.1, An Act to Provide Reconciliation Pursuant to Titles II and V of the Concurrent Resolution on the Budget for Fiscal Year 2018, more commonly known as the Tax Cuts and Jobs Act (TCJA) from accumulated other comprehensive income to retained earnings. Tax effects that are stranded in accumulated other comprehensive income for reasons other than the TCJA may not be reclassified. This update requires additional disclosures on whether an entity elects to reclassify the stranded tax effects and its policy for releasing tax effects from accumulated other comprehensive income. This guidance may be applied in the period of adoption or retrospectively to each period in which the effect of the change in federal income tax rate in the TCJA is recognized. This update is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018; however, early adoption is permitted. January 1, 2019 We intended to early adopt this update effective January 1, 2018, but have chosen to now continue our evaluation of this potential adoption. If we elect to adopt this update, we will reclassify stranded tax effects resulting from the enactment of the TCJA from accumulated other comprehensive income to retained earnings and will apply the amendments as of January 1 in the year the election is made. In addition, we will be required to expand our disclosures. ASC 310 "Receivables - Nonrefundable Fees and Other Costs" This update shortens the amortization period to the earliest call date for certain callable debt securities held at a premium. This update does not impact securities held at a discount. The guidance is to be applied using a modified retrospective approach, with early adoption permitted. January 1, 2019 We have not yet determined the expected impact on our financial position or results of operations. ASC 842 "Leases" This update changes the accounting for leases, requiring lessees to report most leases on their balance sheets, regardless of whether the lease is classified as a finance lease or an operating lease. For lessees, the initial lease liability is equal to the present value of lease payments, and a corresponding asset, adjusted for certain items, is also recorded. Expense recognition for lessees will remain similar to current accounting requirements for capital and operating leases. For lessors, the guidance modifies the classification criteria and the accounting for sales-type and direct financing leases. The guidance is to be applied using a modified retrospective approach at the beginning of the earliest comparative period presented and early adoption is permitted. January 1, 2019 We have not yet determined the expected impact on our financial position or results of operations. ASC Description Date of Adoption Effect on Financial Statements ASC 326 "Financial Instruments - Credit Losses" This update amends the guidance on the impairment of financial instruments. The update adds an impairment model known as the current expected credit loss model that is based on expected losses rather than incurred losses and will generally result in earlier recognition of allowances for losses. The current expected credit loss model applies to financial instruments such as mortgage loans, fixed maturity securities classified as held-to-maturity, and certain receivables. The update also modifies the other-than-temporary impairment model used for available-for-sale fixed maturity securities such that credit losses are recognized as an allowance rather than as a reduction in the amortized cost of the security. The reversal of previously recognized credit losses on available-for-sale fixed maturity securities is allowed under specified circumstances. Additional disclosures will also be required, including information used to develop the allowance for losses. The guidance is to be applied to most instruments in scope using a modified retrospective approach at the beginning of the earliest comparative period presented with early adoption permitted. For available-for-sale fixed maturity securities, the update is applied prospectively. Other-than-temporary impairment losses recognized on available-for-sale fixed maturity securities prior to adoption of the update cannot be reversed. January 1, 2020 We have not yet determined the expected impact on our financial position or results of operations. ASC 350 "Intangibles - Goodwill and Other" This update eliminates the requirement to calculate the implied fair value of goodwill (the second step in the current two-step test) to measure a goodwill impairment charge. Instead, entities should perform the goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount and recognize an impairment charge for the excess of the carrying amount over the fair value, with the loss not to exceed the total amount of goodwill allocated to that reporting unit. The guidance is to be applied prospectively, with early adoption permitted for goodwill impairment tests performed on testing dates after January 1, 2017. January 1, 2020 The adoption of this update is not expected to have a material effect on our financial position or results of operations.
Accounting Updates Outstanding	Accounting Updates Outstanding: ASC Description Date of Adoption Effect on Financial Statements ASC 220 "Income Statement - Reporting Comprehensive Income" This update allows entities to make an accounting policy election to reclassify the stranded tax effects arising as a result of the recognition of the enactment of the tax bill, H.R.1, An Act to Provide Reconciliation Pursuant to Titles II and V of the Concurrent Resolution on the Budget for Fiscal Year 2018, more commonly known as the Tax Cuts and Jobs Act (TCJA) from accumulated other comprehensive income to retained earnings. Tax effects that are stranded in accumulated other comprehensive income for reasons other than the TCJA may not be reclassified. This update requires additional disclosures on whether an entity elects to reclassify the stranded tax effects and its policy for releasing tax effects from accumulated other comprehensive income. This guidance may be applied in the period of adoption or retrospectively to each period in which the effect of the change in federal income tax rate in the TCJA is recognized. This update is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018; however, early adoption is permitted. January 1, 2019 We intended to early adopt this update effective January 1, 2018, but have chosen to now continue our evaluation of this potential adoption. If we elect to adopt this update, we will reclassify stranded tax effects resulting from the enactment of the TCJA from accumulated other comprehensive income to retained earnings and will apply the amendments as of January 1 in the year the election is made. In addition, we will be required to expand our disclosures. ASC 310 "Receivables - Nonrefundable Fees and Other Costs" This update shortens the amortization period to the earliest call date for certain callable debt securities held at a premium. This update does not impact securities held at a discount. The guidance is to be applied using a modified retrospective approach, with early adoption permitted. January 1, 2019 We have not yet determined the expected impact on our financial position or results of operations. ASC 842 "Leases" This update changes the accounting for leases, requiring lessees to report most leases on their balance sheets, regardless of whether the lease is classified as a finance lease or an operating lease. For lessees, the initial lease liability is equal to the present value of lease payments, and a corresponding asset, adjusted for certain items, is also recorded. Expense recognition for lessees will remain similar to current accounting requirements for capital and operating leases. For lessors, the guidance modifies the classification criteria and the accounting for sales-type and direct financing leases. The guidance is to be applied using a modified retrospective approach at the beginning of the earliest comparative period presented and early adoption is permitted. January 1, 2019 We have not yet determined the expected impact on our financial position or results of operations. ASC Description Date of Adoption Effect on Financial Statements ASC 326 "Financial Instruments - Credit Losses" This update amends the guidance on the impairment of financial instruments. The update adds an impairment model known as the current expected credit loss model that is based on expected losses rather than incurred losses and will generally result in earlier recognition of allowances for losses. The current expected credit loss model applies to financial instruments such as mortgage loans, fixed maturity securities classified as held-to-maturity, and certain receivables. The update also modifies the other-than-temporary impairment model used for available-for-sale fixed maturity securities such that credit losses are recognized as an allowance rather than as a reduction in the amortized cost of the security. The reversal of previously recognized credit losses on available-for-sale fixed maturity securities is allowed under specified circumstances. Additional disclosures will also be required, including information used to develop the allowance for losses. The guidance is to be applied to most instruments in scope using a modified retrospective approach at the beginning of the earliest comparative period presented with early adoption permitted. For available-for-sale fixed maturity securities, the update is applied prospectively. Other-than-temporary impairment losses recognized on available-for-sale fixed maturity securities prior to adoption of the update cannot be reversed. January 1, 2020 We have not yet determined the expected impact on our financial position or results of operations. ASC 350 "Intangibles - Goodwill and Other" This update eliminates the requirement to calculate the implied fair value of goodwill (the second step in the current two-step test) to measure a goodwill impairment charge. Instead, entities should perform the goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount and recognize an impairment charge for the excess of the carrying amount over the fair value, with the loss not to exceed the total amount of goodwill allocated to that reporting unit. The guidance is to be applied prospectively, with early adoption permitted for goodwill impairment tests performed on testing dates after January 1, 2017. January 1, 2020 The adoption of this update is not expected to have a material effect on our financial position or results of operations.

Fair Values of Financial Instru

Fair Values of Financial Instruments	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Fair Values of Financial Instruments	Fair Value Measurements for Financial Instruments Carried at Fair Value We report fixed maturity securities, which are classified as available-for-sale securities, derivative financial instruments, and unrestricted equity securities at fair value in our consolidated balance sheets. We report our investments in private equity partnerships at our share of the partnerships' net asset value (NAV) as a practical expedient for fair value. The degree of judgment utilized in measuring the fair value of financial instruments generally correlates to the level of pricing observability. Financial instruments with readily available active quoted prices or for which fair value can be measured from actively quoted prices in active markets generally have more pricing observability and less judgment utilized in measuring fair value. An active market for a financial instrument is a market in which transactions for an asset or a similar asset occur with sufficient frequency and volume to provide pricing information on an ongoing basis. A quoted price in an active market provides the most reliable evidence of fair value and should be used to measure fair value whenever available. Conversely, financial instruments rarely traded or not quoted have less observability and are measured at fair value using valuation techniques that require more judgment. Pricing observability is generally impacted by a number of factors, including the type of financial instrument, whether the financial instrument is new to the market and not yet established, the characteristics specific to the transaction, and overall market conditions. We classify financial instruments in accordance with a fair value hierarchy consisting of three levels based on the observability of valuation inputs: • Level 1 - the highest category of the fair value hierarchy classification wherein inputs are unadjusted and represent quoted prices in active markets for identical assets or liabilities at the measurement date. • Level 2 - valued using inputs (other than prices included in Level 1) that are either directly or indirectly observable for the asset or liability through correlation with market data at the measurement date and for the duration of the instrument's anticipated life. • Level 3 - the lowest category of the fair value hierarchy and reflects the judgment of management regarding what market participants would use in pricing assets or liabilities at the measurement date. Financial assets and liabilities categorized as Level 3 are generally those that are valued using unobservable inputs to extrapolate an estimated fair value. Valuation Methodologies of Financial Instruments Measured at Fair Value Valuation techniques used for assets and liabilities accounted for at fair value are generally categorized into three types. The market approach uses prices and other relevant information from market transactions involving identical or comparable assets or liabilities. The income approach converts future amounts, such as cash flows or earnings, to a single present amount, or a discounted amount. The cost approach is based upon the amount that currently would be required to replace the service capacity of an asset, or the current replacement cost. We use valuation techniques that are appropriate in the circumstances and for which sufficient data are available that can be obtained without undue cost and effort. In some cases, a single valuation technique will be appropriate (for example, when valuing an asset or liability using quoted prices in an active market for identical assets or liabilities). In other cases, multiple valuation techniques will be appropriate. If we use multiple valuation techniques to measure fair value, we evaluate and weigh the results, as appropriate, considering the reasonableness of the range indicated by those results. A fair value measurement is the point within that range that is most representative of fair value in the circumstances. The selection of the valuation method(s) to apply considers the definition of an exit price and depends on the nature of the asset or liability being valued. For assets and liabilities accounted for at fair value, we generally use valuation techniques consistent with the market approach, and to a lesser extent, the income approach. We believe the market approach provides more observable data than the income approach, considering the type of investments we hold. Our fair value measurements could differ significantly based on the valuation technique and available inputs. When using a pricing service, we obtain the vendor's pricing documentation to ensure we understand their methodologies. We periodically review and approve the selection of our pricing vendors to ensure we are in agreement with their current methodologies. When markets are less active, brokers may rely more on models with inputs based on the information available only to the broker. Our internal investment management professionals, which include portfolio managers and analysts, monitor securities priced by brokers and evaluate their prices for reasonableness based on benchmarking to available primary and secondary market information. In weighing a broker quote as an input to fair value, we place less reliance on quotes that do not reflect the result of market transactions. We also consider the nature of the quote, particularly whether the quote is a binding offer. If prices in an inactive market do not reflect current prices for the same or similar assets, adjustments may be necessary to arrive at fair value. When relevant market data is unavailable, which may be the case during periods of market uncertainty, the income approach can, in suitable circumstances, provide a more appropriate fair value. During 2018 , we have applied valuation approaches and techniques on a consistent basis to similar assets and liabilities and consistent with those approaches and techniques used at year end 2017 . Fixed Maturity and Equity Securities We use observable and unobservable inputs in measuring the fair value of our fixed maturity and equity securities. For securities categorized as Level 1, fair values equal active Trade Reporting and Compliance Engine (TRACE) pricing or unadjusted broker market maker prices. For securities categorized as Level 2 or Level 3, inputs that may be used in valuing each class of securities at any given time period are presented as follows. Actual inputs used to determine fair values will vary for each reporting period depending on the availability of inputs which may, at times, be affected by the lack of market liquidity. Level 2 Level 3 Instrument Observable Inputs Unobservable Inputs United States Government and Government Agencies and Authorities Valuation Method Principally the market approach Not applicable Valuation Techniques / Inputs Prices obtained from external pricing services States, Municipalities, and Political Subdivisions Valuation Method Principally the market approach Principally the market approach Valuation Techniques / Inputs Prices obtained from external pricing services Analysis of similar bonds, adjusted for comparability Relevant reports issued by analysts and rating agencies Non-binding broker quotes Audited financial statements Security and issuer level spreads Foreign Governments Valuation Method Principally the market approach Principally the market approach Valuation Techniques / Inputs Prices obtained from external pricing services Analysis of similar bonds, adjusted for comparability Non-binding broker quotes Non-binding broker quotes Call provisions Security and issuer level spreads Level 2 Level 3 Instrument Observable Inputs Unobservable Inputs Public Utilities Valuation Method Principally the market and income approaches Principally the market and income approaches Valuation Techniques / Inputs TRACE pricing Change in benchmark reference Prices obtained from external pricing services Analysis of similar bonds, adjusted for comparability Non-binding broker quotes Discount for size - illiquidity Benchmark yields Non-binding broker quotes Transactional data for new issuances and secondary trades Lack of marketability Security cash flows and structures Security and issuer level spreads Recent issuance / supply Volatility of credit Matrix pricing Matrix pricing Security and issuer level spreads Security creditor ratings/maturity/capital structure/optionality Public covenants Comparative bond analysis Relevant reports issued by analysts and rating agencies Audited financial statements Mortgage/Asset-Backed Securities Valuation Method Principally the market and income approaches Principally the market approach Valuation Techniques / Inputs Prices obtained from external pricing services Analysis of similar bonds, adjusted for comparability Non-binding broker quotes Non-binding broker quotes Security cash flows and structures Security and issuer level spreads Underlying collateral Prepayment speeds/loan performance/delinquencies Relevant reports issued by analysts and rating agencies Audited financial statements Level 2 Level 3 Instrument Observable Inputs Unobservable Inputs All Other Corporate Bonds Valuation Method Principally the market and income approaches Principally the market and income approaches Valuation Techniques / Inputs TRACE pricing Change in benchmark reference Prices obtained from external pricing services Analysis of similar bonds, adjusted for comparability Non-binding broker quotes Discount for size - illiquidity Benchmark yields Non-binding broker quotes Transactional data for new issuances and secondary trades Lack of marketability Security cash flows and structures Security and issuer level spreads Recent issuance / supply Volatility of credit Matrix pricing Matrix pricing Security and issuer level spreads Security creditor ratings/maturity/capital structure/optionality Public covenants Comparative bond analysis Relevant reports issued by analysts and rating agencies Audited financial statements Redeemable Preferred Stocks Valuation Method Principally the market approach Principally the market approach Valuation Techniques / Inputs Non-binding broker quotes Non-binding broker quotes Benchmark yields Comparative bond analysis Call provisions Relevant reports issued by analysts and rating agencies Audited financial statements Equity Securities Valuation Method Principally the market approach Principally the market and income approaches Valuation Techniques / Inputs Prices obtained from external pricing services Financial statement analysis Non-binding broker quotes Non-binding broker quotes The management of our investment portfolio includes establishing pricing policy and reviewing the reasonableness of sources and inputs used in developing pricing. We review all prices obtained to ensure they are consistent with a variety of observable market inputs and to verify the validity of a security's price. In the event we receive a vendor's market price that does not appear reasonable based on our market analysis, we may challenge the price and request further information about the assumptions and methodologies used by the vendor to price the security. We may change the vendor price based on a better data source such as an actual trade. We also review all price changes from the prior month which fall outside a predetermined corridor. The overall valuation process for determining fair values may include adjustments to valuations obtained from our pricing sources when they do not represent a valid exit price. These adjustments may be made when, in our judgment and considering our knowledge of the financial conditions and industry in which the issuer operates, certain features of the financial instrument require that an adjustment be made to the value originally obtained from our pricing sources. These features may include the complexity of the financial instrument, the market in which the financial instrument is traded, counterparty credit risk, credit structure, concentration, or liquidity. Additionally, an adjustment to the price derived from a model typically reflects our judgment of the inputs that other participants in the market for the financial instrument being measured at fair value would consider in pricing that same financial instrument. In the event an asset is sold, we test the validity of the fair value determined by our valuation techniques by comparing the selling price to the fair value determined for the asset in the immediately preceding month end reporting period. Certain of our investments do not have readily determinable market prices and/or observable inputs or may at times be affected by the lack of market liquidity. For these securities, we use internally prepared valuations combining matrix pricing with vendor purchased software programs, including valuations based on estimates of future profitability, to estimate the fair value. Additionally, we may obtain prices from independent third-party brokers to aid in establishing valuations for certain of these securities. Key assumptions used by us to determine fair value for these securities include risk free interest rates, risk premiums, performance of underlying collateral (if any), and other factors involving significant assumptions which may or may not reflect those of an active market. The parameters and inputs used to validate a price on a security may be adjusted for assumptions about risk and current market conditions on a quarter to quarter basis, as certain features may be more significant drivers of valuation at the time of pricing. Changes to inputs in valuations are not changes to valuation methodologies; rather, the inputs are modified to reflect direct or indirect impacts on asset classes from changes in market conditions. At March 31, 2018 , 20.7 percent of our fixed maturity securities were valued using active trades from TRACE pricing or broker market maker prices for which there was current market activity in that specific security (comparable to receiving one binding quote). The prices obtained were not adjusted, and the assets were classified as Level 1. The remaining 79.3 percent of our fixed maturity securities were valued based on non-binding quotes or other observable and unobservable inputs, as discussed below: • 66.0 percent of our fixed maturity securities were valued based on prices from pricing services that generally use observable inputs such as prices for securities or comparable securities in active markets in their valuation techniques. These assets were classified as Level 2. • 2.4 percent of our fixed maturity securities were valued based on one or more non-binding broker quotes, if validated by observable market data, or on TRACE prices for identical or similar assets absent current market activity. When only one price is available, it is used if observable inputs and analysis confirms that it is appropriate. These assets, for which we were able to validate the price using other observable market data, were classified as Level 2. • 10.9 percent of our fixed maturity securities were valued based on prices of comparable securities, matrix pricing, market models, and/or internal models or were valued based on non-binding quotes with no other observable market data. These assets were classified as either Level 2 or Level 3, with the categorization dependent on whether there was other observable market data. Derivatives Fair values for derivatives other than embedded derivatives in modified coinsurance arrangements are based on market quotes or pricing models and represent the net amount of cash we would have paid or received if the contracts had been settled or closed as of the last day of the period. We analyze credit default swap spreads relative to the average credit spread embedded within the LIBOR-setting syndicate in determining the effect of credit risk on our derivatives' fair values. If net counterparty credit risk for a derivative asset is determined to be material and is not adequately reflected in the LIBOR-based fair value obtained from our pricing sources, we adjust the valuations obtained from our pricing sources. For purposes of valuing net counterparty risk, we measure the fair value of a group of financial assets and financial liabilities on the basis of the price that would be received to sell a net long position or transfer a net short position for a particular risk exposure in an orderly transaction between market participants at the measurement date under current market conditions. In regard to our own credit risk component, we adjust the valuation of derivative liabilities wherein the counterparty is exposed to our credit risk when the LIBOR-based valuation of our derivatives obtained from pricing sources does not effectively include an adequate credit component for our own credit risk. Fair values for our embedded derivative in a modified coinsurance arrangement are estimated using internal pricing models and represent the hypothetical value of the duration mismatch of assets and liabilities, interest rate risk, and third party credit risk embedded in the modified coinsurance arrangement. We consider transactions in inactive markets to be less representative of fair value. We use all available observable inputs when measuring fair value, but when significant unobservable inputs are used, we classify these assets or liabilities as Level 3. Private Equity Partnerships Our private equity partnerships represent funds that are primarily invested in private credit, private equity and real assets, as described below. Distributions received from the funds arise from income generated by the underlying investments as well as the liquidation of the underlying investments. There is generally not a public market for these investments. The following table presents additional information about our private equity partnerships as of March 31, 2018, including commitments for additional investments which may or may not be funded: Investment Category Fair Value Redemption Term / Redemption Notice Unfunded Commitments (in millions of dollars) (in millions of dollars) Private Credit (a) $ 165.5 Not redeemable $ 116.1 22.4 Initial 2 year lock on each new investment / Quarterly after 2 year lock with 90 days notice 2.0 Total Private Credit 187.9 118.1 Private Equity (b) 101.3 Not redeemable 106.2 Real Assets (c) 93.9 Not redeemable 83.0 30.2 Quarterly / 90 days notice — Total Real Assets 124.1 83.0 Total Partnerships $ 413.3 $ 307.3 (a) Private Credit - The limited partnerships described in this category employ various investment strategies, generally providing direct lending or other forms of debt financing including first-lien, second-lien, mezzanine, and subordinated loans. The limited partnerships have credit exposure to corporates, physical assets, and/or financial assets within variety of industries (including manufacturing, healthcare, energy, business services, technology, materials, and retail) in North America and, to a lesser extent, outside of North America. Unless specifically disclosed in the table above, these limited partnerships do not allow for redemptions. As of March 31, 2018 , the estimated remaining life of the investments that do not allow for redemptions is approximately 30 percent in the next 3 years, 42 percent during the period from 3 to 5 years, 23 percent during the period from 5 to 10 years, and 5 percent during the period from 10 to 15 years. (b) Private Equity - The limited partnerships described in this category employ various strategies generally investing in controlling or minority control equity positions directly in companies and/or assets across various industries (including manufacturing, healthcare, energy, business services, technology, materials, and retail), primarily in private markets within North America and, to a lesser extent, outside of North America. Unless specifically disclosed in the table above, these limited partnerships do not allow for redemptions. As of March 31, 2018 , the estimated remaining life of the investments that do not allow for redemptions is approximately 39 percent in the next 3 years, 31 percent during the period from 3 to 5 years, 28 percent during the period from 5 to 10 years, and 2 percent during the period from 10 to 15 years. (c) Real Assets - The limited partnerships described in this category employ various strategies, which include investing in the equity and/or debt financing of physical assets, including infrastructure (energy, power, water/wastewater, communications), transportation (including airports, ports, toll roads, aircraft, railcars) and real estate in North America, Europe, South America, and Asia. Unless specifically disclosed in the table above, these limited partnerships do not allow for redemption. As of March 31, 2018 , the estimated remaining life of the investments that do not allow for redemptions is approximately 3 percent in the next 3 years, 95 percent during the period from 5 to 10 years, and 2 percent during the period from 10 to 15 years. The following tables present information about assets and liabilities measured at fair value on a recurring basis by fair value level, based on the observability of the inputs used: March 31, 2018 Level 1 Level 2 Level 3 NAV Total (in millions of dollars) Assets Fixed Maturity Securities United States Government and Government Agencies and Authorities $ 495.3 $ 1,051.0 $ — $ — $ 1,546.3 States, Municipalities, and Political Subdivisions — 2,321.6 35.7 — 2,357.3 Foreign Governments — 841.4 32.3 — 873.7 Public Utilities 574.1 7,123.4 284.2 — 7,981.7 Mortgage/Asset-Backed Securities — 1,842.3 0.5 — 1,842.8 All Other Corporate Bonds 8,127.2 20,742.6 987.2 — 29,857.0 Redeemable Preferred Stocks — 19.1 22.2 — 41.3 Total Fixed Maturity Securities 9,196.6 33,941.4 1,362.1 — 44,500.1 Other Long-term Investments Derivatives Foreign Exchange Contracts — 21.5 — — 21.5 Equity Securities 24.9 46.4 1.1 — 72.4 Private Equity Partnerships — — — 413.3 413.3 Total Other Long-term Investments 24.9 67.9 1.1 413.3 507.2 Total Recurring Fair Value Measurements Assets $ 9,221.5 $ 34,009.3 $ 1,363.2 $ 413.3 $ 45,007.3 Liabilities Other Liabilities Derivatives Interest Rate Swaps $ — $ 7.2 $ — $ — $ 7.2 Foreign Exchange Contracts — 44.2 — — 44.2 Embedded Derivative in Modified Coinsurance Arrangement — — 17.6 — 17.6 Total Derivatives — 51.4 17.6 — 69.0 Total Recurring Fair Value Measurements Liabilities $ — $ 51.4 $ 17.6 $ — $ 69.0 December 31, 2017 Level 1 Level 2 Level 3 NAV Total (in millions of dollars) Assets Fixed Maturity Securities United States Government and Government Agencies and Authorities $ 460.1 $ 1,022.4 $ — $ — $ 1,482.5 States, Municipalities, and Political Subdivisions — 2,336.9 — — 2,336.9 Foreign Governments — 863.9 — — 863.9 Public Utilities 154.2 7,874.6 207.7 — 8,236.5 Mortgage/Asset-Backed Securities — 1,973.6 — — 1,973.6 All Other Corporate Bonds 3,556.1 25,816.2 1,150.1 — 30,522.4 Redeemable Preferred Stocks — 19.2 22.8 — 42.0 Total Fixed Maturity Securities 4,170.4 39,906.8 1,380.6 — 45,457.8 Other Long-term Investments Derivatives Foreign Exchange Contracts — 19.5 — — 19.5 Equity Securities 0.2 10.4 1.1 — 11.7 Private Equity Partnerships — — — 407.2 407.2 Total Other Long-term Investments 0.2 29.9 1.1 407.2 438.4 Total Recurring Fair Value Measurements Assets $ 4,170.6 $ 39,936.7 $ 1,381.7 $ 407.2 $ 45,896.2 Liabilities Other Liabilities Derivatives Interest Rate Swaps $ — $ 5.1 $ — $ — $ 5.1 Foreign Exchange Contracts — 46.9 — — 46.9 Credit Default Swaps — 0.2 — — 0.2 Embedded Derivative in Modified Coinsurance Arrangement — — 15.9 — 15.9 Total Derivatives — 52.2 15.9 — 68.1 Total Recurring Fair Value Measurements Liabilities $ — $ 52.2 $ 15.9 $ — $ 68.1 Transfers of assets between Level 1 and Level 2 are as follows: Three Months Ended March 31 2018 2017 Transfers into Level 1 from Level 2 from Level 1 from Level 2 from (in millions of dollars) Fixed Maturity Securities United States Government and Government Agencies and Authorities $ 40.1 $ — $ 54.5 $ 180.0 Public Utilities 528.5 62.0 1,103.9 57.5 All Other Corporate Bonds 5,274.9 953.3 6,317.0 1,075.3 Total Fixed Maturity Securities $ 5,843.5 $ 1,015.3 $ 7,475.4 $ 1,312.8 Equity Securities $ 24.2 $ 14.9 $ — $ — Transfers between Level 1 and Level 2 occurred due to the change in availability of either a TRACE or broker market maker price. Depending on current market conditions, the availability of these Level 1 prices can vary from period to period. For fair value measurements of financial instruments that were transferred either into or out of Level 1 or 2, we reflect the transfers using the fair value at the beginning of the period. Changes in assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) are as follows: Three Months Ended March 31, 2018 Total Realized and Unrealized Investment Gains (Losses) Included in Level 3 Transfers Fair Value Beginning of Period Earnings Other Comprehensive Income or Loss Purchases Sales Into Out of Fair Value End of Period (in millions of dollars) Fixed Maturity Securities States, Municipalities, and Political Subdivisions $ — $ — $ (0.8 ) $ — $ (0.1 ) $ 36.6 $ — $ 35.7 Foreign Governments — — (0.4 ) — — 32.7 — 32.3 Public Utilities 207.7 — (6.7 ) — (1.1 ) 200.9 (116.6 ) 284.2 Mortgage/Asset-Backed Securities — — — — — 0.5 — 0.5 All Other Corporate Bonds 1,150.1 — (22.1 ) — (11.7 ) 466.0 (595.1 ) 987.2 Redeemable Preferred Stocks 22.8 — (0.6 ) — — — — 22.2 Total Fixed Maturity Securities 1,380.6 — (30.6 ) — (12.9 ) 736.7 (711.7 ) 1,362.1 Equity Securities 1.1 — — — — — — 1.1 Embedded Derivative in Modified Coinsurance Arrangement (15.9 ) (1.7 ) — — — — — (17.6 ) Three Months Ended March 31, 2017 Total Realized and Unrealized Investment Gains (Losses) Included in Level 3 Transfers Fair Value Beginning of Period Earnings Other Comprehensive Income or Loss Purchases Sales Into Out of Fair Value End of Period (in millions of dollars) Fixed Maturity Securities States, Municipalities, and Political Subdivisions $ 89.5 $ — $ — $ — $ — $ 0.4 $ (89.5 ) $ 0.4 Public Utilities 265.3 — 1.8 8.0 — 112.6 (75.2 ) 312.5 All Other Corporate Bonds 1,459.7 — 13.1 — (36.4 ) 143.9 (657.3 ) 923.0 Redeemable Preferred Stocks 23.2 — (0.3 ) — — — — 22.9 Total Fixed Maturity Securities 1,837.7 — 14.6 8.0 (36.4 ) 256.9 (822.0 ) 1,258.8 Equity Securities 1.2 — — — — — — 1.2 Embedded Derivative in Modified Coinsurance Arrangement (46.7 ) 8.6 — — — — — (38.1 ) Realized and unrealized investment gains and losses presented in the preceding tables represent gains and losses only for the time during which the applicable financial instruments were classified as Level 3. The transfers between levels resulted primarily from a change in observability of three inputs used to determine fair values of the securities transferred: (1) transactional data for new issuance and secondary trades, (2) broker/dealer quotes and pricing, primarily related to changes in the level of activity in the market and whether the market was considered orderly, and (3) comparable bond metrics from which to perform an analysis. For fair value measurements of financial instruments that were transferred either into or out of Level 3, we reflect the transfers using the fair value at the beginning of the period. We believe this allows for greater transparency, as all changes in fair value that arise during the reporting period of the transfer are disclosed as a component of our Level 3 reconciliation. Gains (losses) which are included in earnings and are attributable to the change in fair value of assets or liabilities valued using significant unobservable inputs and still held at period end were $(1.7) million and $8.6 million for the three months ended March 31, 2018 and 2017 , respectively. These amounts relate entirely to the change in fair value of an embedded derivative in a modified coinsurance arrangement and are reported as a component of realized investment gains and losses. The table below provides quantitative information regarding the significant unobservable inputs used in Level 3 fair value measurements derived from internal models. Certain securities classified as Level 3 are excluded from the table below due to limitations in our ability to obtain the underlying inputs used by external pricing sources. March 31, 2018 Fair Value Valuation Method Unobservable Input Range/Weighted Average (in millions of dollars) Fixed Maturity Securities All Other Corporate Bonds - Private $ 276.6 Market Approach Comparability Adjustment (a) 1.00% - 1.25% / 1.04% Equity Securities - Private $ 1.1 Market Approach Market Convention (d) Priced at Cost or Owner's Equity Embedded Derivative in Modified Coinsurance Arrangement $ (17.6 ) Discounted Cash Flows Projected Liability Cash Flows (e) Actuarial Assumptions December 31, 2017 Fair Value Valuation Method Unobservable Input Range/Weighted Average (in millions of dollars) Fixed Maturity Securities All Other Corporate Bonds - Private $ 244.4 Market Approach Comparability Adjustment (a) 0.20% - 0.20% / 0.20% Equity Securities - Private $ 1.1 Market Approach Market Convention (d) Priced at Cost or Owner's Equity Embedded Derivative in Modified Coinsurance Arrangement $ (15.9 ) Discounted Cash Flows Projected Liability Cash Flows (e) Actuarial Assumptions (a) Represents basis point adjustments for changes in benchmark spreads associated with various industry sectors (b) Represents basis point adjustments to apply a discount due to the illiquidity of an investment (c) Represents basis point adjustments for credit-specific factors (d) Represents a decision to price based on par value, cost, or owner's equity when limited data is available (e) Represents various actuarial assumptions required to derive the liability cash flows including incidence, termination, and lapse rates Isolated increases in unobservable inputs other than market convention will result in a lower fair value measurement, whereas isolated decreases will result in a higher fair value measurement. The unobservable input for market convention is not sensitive to input movements. The projected liability cash flows used in the fair value measurement of our Level 3 embedded derivative are based on expected claim payments. If claim payments increase, the projected liability cash flows will increase, resulting in a decrease in the fair value of the embedded derivative. Decreases in projected liability cash flows will result in an increase in the fair value of the embedded derivative. Fair Value Measurements for Financial Instruments Not Carried at Fair Value The methods and assumptions used to estimate fair values of financial instruments not carried at fair value are discussed as follows. Mortgage Loans: Fair values are estimated using discounted cash flow analyses and interest rates currently being offered for similar loans to borrowers with similar credit ratings and maturities. Loans with similar characteristics are aggregated for purposes of the calculations. Policy Loans: Fair values for policy loans, net of reinsurance ceded, are estimated using discounted cash flow analyses and interest rates currently being offered to policyholders with similar policies. Carrying amounts for ceded policy loans, which equal $3,306.8 million and $3,307.5 million as of March 31, 2018 and December 31, 2017 , respectively, approximate fair value and are reported on a gross basis in our consolidated balance sheets. A change in interest rates for ceded policy loans will not impact our financial position because the benefits and risks are fully ceded to reinsuring counterparties. Miscellaneous Long-term Investments: Carrying amounts for tax credit partnerships equal the unamortized balance of our contractual commitments and approximate fair value. Our shares of FHLB common stock are carried at cost, which approximates fair value. Long-term Debt: Fair values for long-term debt are obtained from independent pricing services or discounted cash flow analyses based on current incremental borrowing rates for similar types of borrowing arrangements. FHLB Funding Agreements: Funding agreements with the FHLB represent cash advances used for the purpose of investing in fixed maturity securities. Carrying amounts approximate fair value. Unfunded Commitments to Investment Partnerships: Unfunded equity commitments represent amounts that we have committed to fund certain investment partnerships. These commitments are legally binding, subject to the partnerships meeting specified conditions. Carrying amounts of these financial instruments approximate fair value. The following table presents the carrying amounts and es

Investments

Investments	3 Months Ended
	Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Investments	Fixed Maturity Securities At March 31, 2018 and December 31, 2017 , all fixed maturity securities were classified as available-for-sale. The amortized cost and fair values of securities by security type are shown as follows: March 31, 2018 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value (in millions of dollars) United States Government and Government Agencies and Authorities $ 1,411.0 $ 146.4 $ 11.1 $ 1,546.3 States, Municipalities, and Political Subdivisions 2,016.6 343.2 2.5 2,357.3 Foreign Governments 692.6 181.9 0.8 873.7 Public Utilities 6,919.6 1,089.7 27.6 7,981.7 Mortgage/Asset-Backed Securities 1,776.4 81.1 14.7 1,842.8 All Other Corporate Bonds 27,296.7 2,796.7 236.4 29,857.0 Redeemable Preferred Stocks 39.0 2.3 — 41.3 Total Fixed Maturity Securities $ 40,151.9 $ 4,641.3 $ 293.1 $ 44,500.1 December 31, 2017 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value (in millions of dollars) United States Government and Government Agencies and Authorities $ 1,311.1 $ 176.1 $ 4.7 $ 1,482.5 States, Municipalities, and Political Subdivisions 1,942.8 395.4 1.3 2,336.9 Foreign Governments 673.0 191.3 0.4 863.9 Public Utilities 6,952.7 1,296.4 12.6 8,236.5 Mortgage/Asset-Backed Securities 1,873.2 105.1 4.7 1,973.6 All Other Corporate Bonds 26,988.7 3,633.5 99.8 30,522.4 Redeemable Preferred Stocks 39.0 3.0 — 42.0 Total Fixed Maturity Securities $ 39,780.5 $ 5,800.8 $ 123.5 $ 45,457.8 The following charts indicate the length of time our fixed maturity securities have been in a gross unrealized loss position. March 31, 2018 Less Than 12 Months 12 Months or Greater Fair Value Gross Unrealized Loss Fair Value Gross Unrealized Loss (in millions of dollars) United States Government and Government Agencies and Authorities $ 178.4 $ 8.0 $ 57.5 $ 3.1 States, Municipalities, and Political Subdivisions 134.3 1.4 24.2 1.1 Foreign Governments 38.8 0.8 — — Public Utilities 400.7 17.8 152.2 9.8 Mortgage/Asset-Backed Securities 366.3 9.4 114.1 5.3 All Other Corporate Bonds 5,376.8 149.7 745.2 86.7 Total Fixed Maturity Securities $ 6,495.3 $ 187.1 $ 1,093.2 $ 106.0 December 31, 2017 Less Than 12 Months 12 Months or Greater Fair Value Gross Unrealized Loss Fair Value Gross Unrealized Loss (in millions of dollars) United States Government and Government Agencies and Authorities $ 157.9 $ 3.0 $ 58.8 $ 1.8 States, Municipalities, and Political Subdivisions 45.7 0.5 22.3 0.7 Foreign Governments 13.2 0.4 — — Public Utilities 213.2 7.9 133.5 4.7 Mortgage/Asset-Backed Securities 252.5 1.4 144.7 3.3 All Other Corporate Bonds 1,355.1 26.8 785.2 73.0 Total Fixed Maturity Securities $ 2,037.6 $ 40.0 $ 1,144.5 $ 83.5 The following is a distribution of the maturity dates for fixed maturity securities. The maturity dates have not been adjusted for possible calls or prepayments. March 31, 2018 Total Amortized Cost Unrealized Gain Position Unrealized Loss Position Gross Gain Fair Value Gross Loss Fair Value (in millions of dollars) 1 year or less $ 1,694.9 $ 23.4 $ 1,645.0 $ 4.1 $ 69.2 Over 1 year through 5 years 5,665.0 372.0 5,680.4 22.3 334.3 Over 5 years through 10 years 12,273.8 903.8 9,124.4 138.1 3,915.1 Over 10 years 18,741.8 3,261.0 19,099.4 113.9 2,789.5 38,375.5 4,560.2 35,549.2 278.4 7,108.1 Mortgage/Asset-Backed Securities 1,776.4 81.1 1,362.4 14.7 480.4 Total Fixed Maturity Securities $ 40,151.9 $ 4,641.3 $ 36,911.6 $ 293.1 $ 7,588.5 December 31, 2017 Total Amortized Cost Unrealized Gain Position Unrealized Loss Position Gross Gain Fair Value Gross Loss Fair Value (in millions of dollars) 1 year or less $ 1,625.1 $ 30.9 $ 1,638.8 $ 3.1 $ 14.1 Over 1 year through 5 years 5,579.9 453.6 5,810.9 18.5 204.1 Over 5 years through 10 years 12,091.1 1,169.8 11,916.5 53.2 1,291.2 Over 10 years 18,611.2 4,041.4 21,333.1 44.0 1,275.5 37,907.3 5,695.7 40,699.3 118.8 2,784.9 Mortgage/Asset-Backed Securities 1,873.2 105.1 1,576.4 4.7 397.2 Total Fixed Maturity Securities $ 39,780.5 $ 5,800.8 $ 42,275.7 $ 123.5 $ 3,182.1 The following chart depicts an analysis of our fixed maturity security portfolio between investment-grade and below-investment-grade categories as of March 31, 2018: Gross Unrealized Loss Fair Value Gross Unrealized Gain Amount Percent of Total Gross Unrealized Loss (in millions of dollars) Investment-Grade $ 41,233.8 $ 4,527.8 $ 198.7 67.8 % Below-Investment-Grade 3,266.3 113.5 94.4 32.2 Total Fixed Maturity Securities $ 44,500.1 $ 4,641.3 $ 293.1 100.0 % The unrealized losses on investment-grade fixed maturity securities principally relate to changes in interest rates or changes in market or sector credit spreads which occurred subsequent to the acquisition of the securities. Below-investment-grade fixed maturity securities are generally more likely to develop credit concerns than investment-grade securities. At March 31, 2018 , the unrealized losses in our below-investment-grade fixed maturity securities were generally due to credit spreads in certain industries or sectors and, to a lesser extent, credit concerns related to specific securities. For each specific security in an unrealized loss position, we believe that there are positive factors which mitigate credit concerns and that the securities for which we have not recorded an other-than-temporary impairment will recover in value. As of March 31, 2018 , we held 413 individual investment-grade fixed maturity securities and 88 individual below-investment-grade fixed maturity securities that were in an unrealized loss position, of which 141 investment-grade fixed maturity securities and 20 below-investment-grade fixed maturity securities had been in an unrealized loss position continuously for over one year. In determining when a decline in fair value below amortized cost of a fixed maturity security is other than temporary, we evaluate the following factors: • Whether we expect to recover the entire amortized cost basis of the security • Whether we intend to sell the security or will be required to sell the security before the recovery of its amortized cost basis • Whether the security is current as to principal and interest payments • The significance of the decline in value • The time period during which there has been a significant decline in value • Current and future business prospects and trends of earnings • The valuation of the security's underlying collateral • Relevant industry conditions and trends relative to their historical cycles • Market conditions • Rating agency and governmental actions • Bid and offering prices and the level of trading activity • Adverse changes in estimated cash flows for securitized investments • Changes in fair value subsequent to the balance sheet date • Any other key measures for the related security While determining other-than-temporary impairments is a judgmental area, we utilize a formal, well-defined, and disciplined process to monitor and evaluate our fixed income investment portfolio, supported by issuer specific research and documentation as of the end of each period. The process results in a thorough evaluation of problem investments and the recording of losses on a timely basis for investments determined to have an other-than-temporary impairment. We held no fixed maturity securities as of March 31, 2018 or December 31, 2017 for which a portion of an other-than-temporary impairment was recognized in accumulated other comprehensive income. At March 31, 2018 , we had commitments of $213.5 million to fund private placement fixed maturity securities, the amount of which may or may not be funded. Variable Interest Entities We invest in variable interests issued by variable interest entities. These investments include tax credit partnerships, private equity partnerships, and special purpose entities. For those variable interests that are not consolidated in our financial statements, we are not the primary beneficiary because we have neither the power to direct the activities that are most significant to economic performance nor the responsibility to absorb a majority of the expected losses. The determination of whether we are the primary beneficiary is performed at the time of our initial investment and at the date of each subsequent reporting period. As of March 31, 2018 , the carrying amount of our variable interest entity investments that are not consolidated in our financial statements was $532.4 million , comprised of $119.1 million of tax credit partnerships and $413.3 million of private equity partnerships. At December 31, 2017 , the carrying amount of our variable interest entity investments that are not consolidated in our financial statements was $535.4 million , comprised of $128.2 million of tax credit partnerships and $407.2 million of private equity partnerships. These variable interest entity investments are reported as other long-term investments in our consolidated balance sheets. The Company invests in tax credit partnerships primarily for the receipt of income tax credits and tax benefits derived from passive losses on the investments. Amounts recognized in the consolidated statements of income are as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Income Tax Credits $ 10.4 $ 10.4 Amortization, net of tax (7.0 ) (5.8 ) Income Tax Benefit $ 3.4 $ 4.6 Contractually, we are a limited partner in these tax credit partnerships, and our maximum exposure to loss is limited to the carrying value of our investment, which includes $3.7 million of unfunded unconditional commitments at March 31, 2018 . See Note 3 for commitments to fund private equity partnerships. We are the sole beneficiary of a special purpose entity which is consolidated in our financial statements. This entity is a securitized asset trust containing a highly rated bond for principal protection which we contributed into the trust at the time it was established. There are no restrictions on the asset held in this trust, and the trust is free to dispose of the asset at any time. The fair values of the bond were $154.4 million and $154.1 million as of March 31, 2018 and December 31, 2017 , respectively. The bond is reported as a component of fixed maturity securities in our consolidated balance sheets. Mortgage Loans Our mortgage loan portfolio is well diversified by both geographic region and property type to reduce risk of concentration. All of our mortgage loans are collateralized by commercial real estate. When issuing a new loan, our general policy is not to exceed a loan-to-value ratio, or the ratio of the loan balance to the estimated fair value of the underlying collateral, of 75 percent. We update the loan-to-value ratios at least every three years for each loan, and properties undergo a general inspection at least every two years. Our general policy for newly issued loans is to have a debt service coverage ratio greater than 1.25 times on a normalized 25 year amortization period. We update our debt service coverage ratios annually. Mortgage loans by property type and geographic region are presented below. March 31, 2018 December 31, 2017 (in millions of dollars) Carrying Percent of Carrying Percent of Amount Total Amount Total Property Type Apartment $ 380.0 17.3 % $ 360.0 16.3 % Industrial 584.9 26.6 601.2 27.2 Office 674.5 30.6 692.3 31.3 Retail 529.5 24.1 527.6 23.8 Other 31.9 1.4 32.1 1.4 Total $ 2,200.8 100.0 % $ 2,213.2 100.0 % Region New England $ 55.2 2.5 % $ 56.1 2.5 % Mid-Atlantic 154.5 7.0 155.5 7.0 East North Central 291.3 13.2 282.0 12.8 West North Central 208.4 9.5 210.1 9.5 South Atlantic 481.7 21.9 494.4 22.3 East South Central 88.1 4.0 88.8 4.0 West South Central 245.4 11.2 247.4 11.2 Mountain 254.9 11.6 251.2 11.4 Pacific 421.3 19.1 427.7 19.3 Total $ 2,200.8 100.0 % $ 2,213.2 100.0 % We evaluate each of our mortgage loans individually for impairment and assign an internal credit quality rating based on a comprehensive rating system used to evaluate the credit risk of the loan. The factors we use to derive our internal credit ratings may include the following: • Loan-to-value ratio • Debt service coverage ratio based on current operating income • Property location, including regional economics, trends and demographics • Age, condition, and construction quality of property • Current and historical occupancy of property • Lease terms relative to market • Tenant size and financial strength • Borrower's financial strength • Borrower's equity in transaction • Additional collateral, if any Although all available and applicable factors are considered in our analysis, loan-to-value and debt service coverage ratios are the most critical factors in determining whether we will initially issue the loan and also in assigning values and determining impairment. We assign an overall rating to each loan using an internal rating scale of Aa (highest quality) to B (lowest quality). We review and adjust, as needed, our internal credit quality ratings on an annual basis. This review process is performed more frequently for mortgage loans deemed to have a higher risk of delinquency. Mortgage loans, sorted by the applicable credit quality indicators, are as follows: March 31, 2018 December 31, 2017 (in millions of dollars) Internal Rating Aa $ 0.3 $ 0.4 A 481.9 445.7 Baa 1,704.4 1,753.0 Ba 14.2 14.1 Total $ 2,200.8 $ 2,213.2 Loan-to-Value Ratio <= 65% $ 1,082.6 $ 1,101.7 > 65% <= 75% 1,053.8 1,041.6 > 75% <= 85% 40.2 49.3 > 85% 24.2 20.6 Total $ 2,200.8 $ 2,213.2 There were no troubled debt restructurings during the three months ended March 31, 2018 or 2017 . At March 31, 2018 , we held one mortgage loan with a carrying value of $3.2 million that was greater than 90 days past due regarding principal and/or interest payments. At December 31, 2017 , we held no mortgage loans that were greater than 90 days past due regarding principal and/or interest payments. There have been no changes to our accounting policies or methodology from the prior period regarding estimating the allowance for credit losses on our mortgage loans. As of March 31, 2018 and December 31, 2017 , we had no allowance for credit losses, and there was no activity in the allowance for credit losses during the three months ended March 31, 2018 or 2017 . Our average investment in impaired mortgage loans was $1.1 million for the three months ended March 31, 2018 . We did not hold any impaired mortgage loans during the three months ended March 31, 2017, nor did we recognize any interest income on mortgage loans subsequent to impairment for the three months ended March 31, 2018 or 2017 . At March 31, 2018 , we had commitments of $78.9 million to fund certain commercial mortgage loans, the amount of which may or may not be funded. Transfers of Financial Assets To manage our cash position more efficiently, we may enter into repurchase agreements with unaffiliated financial institutions. We generally use repurchase agreements as a means to finance the purchase of invested assets or for short-term general business purposes until projected cash flows become available from our operations or existing investments. Our repurchase agreements are typically outstanding for less than 30 days . We post collateral through our repurchase agreement transactions whereby the counterparty commits to purchase securities with the agreement to resell them to us at a later, specified date. The fair value of collateral posted is generally 102 percent of the cash received. Our investment policy also permits us to lend fixed maturity securities to unaffiliated financial institutions in short-term securities lending agreements. These agreements increase our investment income with minimal risk. Our securities lending policy requires that a minimum of 102 percent of the fair value of the securities loaned be maintained as collateral. We may receive cash and/or securities as collateral under these agreements. Cash received as collateral is typically reinvested in short-term investments. If securities are received as collateral, we are not permitted to sell or re-post them. As of March 31, 2018 , the carrying amount of fixed maturity securities loaned to third parties under our securities lending program was $104.2 million , for which we received collateral in the form of cash and securities of $16.0 million and $94.3 million , respectively. As of December 31, 2017 , the carrying amount of fixed maturity securities loaned to third parties under our securities lending program was $159.2 million , for which we received collateral in the form of cash and securities of $30.5 million and $135.6 million , respectively. We had no outstanding repurchase agreements at March 31, 2018 or December 31, 2017 . The remaining contractual maturities of our securities lending agreements disaggregated by class of collateral pledged are as follows: March 31, 2018 December 31, 2017 Overnight and Continuous (in millions of dollars) United States Government and Government Agencies and Authorities $ 1.0 $ 0.2 Public Utilities — 0.5 All Other Corporate Bonds 15.0 29.8 Total Borrowings 16.0 30.5 Gross Amount of Recognized Liability for Securities Lending Transactions 16.0 30.5 Amounts Related to Agreements Not Included in Offsetting Disclosure Contained Herein $ — $ — Certain of our U.S. insurance subsidiaries are members of regional FHLBs. Membership, which requires that we purchase a minimum amount of FHLB common stock on which we receive dividends, provides access to low-cost funding. Advances received from the FHLB are used for the purchase of fixed maturity securities. Additional common stock purchases may be required, based on the amount of funds we borrow from the FHLBs. The carrying value of common stock owned, collateral posted, and advances received are as follows: March 31, 2018 December 31, 2017 (in millions of dollars) Carrying Value of FHLB Common Stock $ 34.1 $ 34.1 Advances from FHLB 350.0 350.0 Carrying Value of Collateral Posted to FHLB Fixed Maturity Securities $ 268.5 $ 213.3 Commercial Mortgage Loans 321.6 331.5 Total Carrying Value of Collateral Posted to FHLB $ 590.1 $ 544.8 Offsetting of Financial Instruments We enter into master netting agreements with each of our derivatives counterparties. These agreements provide for conditional rights of set-off upon the occurrence of an early termination event. An early termination event is considered a default, and it allows the non-defaulting party to offset its contracts in a loss position against any gain positions or payments due to the defaulting party. Under our agreements, default type events are defined as failure to pay or deliver as contractually agreed, misrepresentation, bankruptcy, or merger without assumption. See Note 5 for further discussion of collateral related to our derivative contracts. We have securities lending agreements with unaffiliated financial institutions that post collateral to us in return for the use of our fixed maturity securities. A right of set-off exists that allows us to keep and apply collateral received in the event of default by the counterparty. Default within a securities lending agreement would typically occur if the counterparty failed to return the securities borrowed from us as contractually agreed. In addition, if we default by not returning collateral received, the counterparty has a right of set-off against our securities or any other amounts due to us. Shown below are our financial instruments that either meet the accounting requirements that allow them to be offset in our balance sheets or that are subject to an enforceable master netting arrangement or similar agreement. Our accounting policy is to not offset these financial instruments in our balance sheets. Net amounts disclosed below have been reduced by the amount of collateral pledged to or received from our counterparties. March 31, 2018 Gross Amount Gross Amount Not of Recognized Gross Amount Net Amount Offset in Balance Sheet Financial Offset in Presented in Financial Cash Net Instruments Balance Sheet Balance Sheet Instruments Collateral Amount (in millions of dollars) Financial Assets: Derivatives $ 21.5 $ — $ 21.5 $ (5.4 ) $ (16.1 ) $ — Securities Lending 104.2 — 104.2 (88.2 ) (16.0 ) — Total $ 125.7 $ — $ 125.7 $ (93.6 ) $ (32.1 ) $ — Financial Liabilities: Derivatives $ 51.4 $ — $ 51.4 $ (39.0 ) $ — $ 12.4 Securities Lending 16.0 — 16.0 (16.0 ) — — Total $ 67.4 $ — $ 67.4 $ (55.0 ) $ — $ 12.4 December 31, 2017 Gross Amount Gross Amount Not of Recognized Gross Amount Net Amount Offset in Balance Sheet Financial Offset in Presented in Financial Cash Net Instruments Balance Sheet Balance Sheet Instruments Collateral Amount (in millions of dollars) Financial Assets: Derivatives $ 19.5 $ — $ 19.5 $ (4.2 ) $ (15.3 ) $ — Securities Lending 159.2 — 159.2 (128.7 ) (30.5 ) — Total $ 178.7 $ — $ 178.7 $ (132.9 ) $ (45.8 ) $ — Financial Liabilities: Derivatives $ 52.2 $ — $ 52.2 $ (42.9 ) $ — $ 9.3 Securities Lending 30.5 — 30.5 (30.5 ) — — Total $ 82.7 $ — $ 82.7 $ (73.4 ) $ — $ 9.3 Net Investment Income Net investment income reported in our consolidated statements of income is as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Fixed Maturity Securities $ 557.1 $ 563.7 Derivatives 15.4 13.4 Mortgage Loans 27.3 25.4 Policy Loans 4.4 4.3 Other Long-term Investments Equity Securities 1 (0.2 ) 0.3 Private Equity Partnerships 1 5.6 5.1 Other 2.1 0.7 Short-term Investments 3.6 1.9 Gross Investment Income 615.3 614.8 Less Investment Expenses 9.6 8.8 Less Investment Income on Participation Fund Account Assets 3.4 3.6 Net Investment Income $ 602.3 $ 602.4 1 The net unrealized loss recognized in net investment income for the first three months of 2018 related to equity securities and private equity partnerships still held at March 31, 2018 was $2.0 million and $1.6 million , respectively. Realized Investment Gain and Loss Realized investment gains and losses are as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Fixed Maturity Securities Gross Gains on Sales $ 2.0 $ 2.4 Gross Losses on Sales (1.4 ) (1.1 ) Other-Than-Temporary Impairment Loss (1.0 ) — Mortgage Loans and Other Invested Assets Gross Gains on Sales — 0.7 Gross Losses on Sales — (0.2 ) Embedded Derivative in Modified Coinsurance Arrangement (1.7 ) 8.6 All Other Derivatives 0.7 (0.2 ) Foreign Currency Transactions (0.8 ) 0.8 Net Realized Investment Gain (Loss) $ (2.2 ) $ 11.0

Derivative Financial Instrument

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 2018
Notes to Financial Statements [Abstract]
Derivative Financial Instruments	Purpose of Derivatives We are exposed to certain risks relating to our ongoing business operations. The primary risks managed by using derivative instruments are interest rate risk, risk related to matching duration for our assets and liabilities, foreign currency risk, and credit risk. Historically, we have utilized current and forward interest rate swaps, current and forward currency swaps, forward treasury locks, currency forward contracts, forward contracts on specific fixed income securities, and credit default swaps. Transactions hedging interest rate risk are primarily associated with our individual and group long-term care and individual and group disability products. All other product portfolios are periodically reviewed to determine if hedging strategies would be appropriate for risk management purposes. We do not use derivative financial instruments for speculative purposes. Derivatives designated as cash flow hedges and used to reduce our exposure to interest rate and duration risk are as follows: • Interest rate swaps are used to hedge interest rate risks and to improve the matching of assets and liabilities. An interest rate swap is an agreement in which we agree with other parties to exchange, at specified intervals, the difference between fixed rate and variable rate interest amounts. We use interest rate swaps to hedge the anticipated purchase of fixed maturity securities thereby protecting us from the potential adverse impact of declining interest rates on the associated policy reserves. We also use interest rate swaps to hedge the potential adverse impact of rising interest rates in anticipation of issuing fixed rate long-term debt. • Forward treasury locks are used to minimize interest rate risk associated with the anticipated purchase or disposal of fixed maturity securities. A forward treasury lock is a derivative contract without an initial investment where we and the counterparty agree to purchase or sell a specific U.S. Treasury bond at a future date at a pre-determined price. Derivatives designated as fair value hedges and used to reduce our exposure to interest rate and duration risk are as follows: • Interest rate swaps are used to effectively convert certain of our fixed rate securities into floating rate securities which are used to fund our floating rate long-term debt. Under these swap agreements, we receive a variable rate of interest and pay a fixed rate of interest. Additionally, we use interest rate swaps to effectively convert certain fixed rate, long-term debt into floating rate long-term debt. Under these swap agreements, we receive a fixed rate of interest and pay a variable rate of interest. Derivatives designated as either cash flow or fair value hedges and used to reduce our exposure to foreign currency risk are as follows: • Foreign currency interest rate swaps are used to hedge the currency risk of certain foreign currency-denominated fixed maturity securities owned for portfolio diversification. Under these swap agreements, we agree to pay, at specified intervals, fixed rate foreign currency-denominated principal and interest payments in exchange for fixed rate payments in the functional currency of the operating segment. Derivatives not designated as hedging instruments and used to reduce our exposure to foreign currency risk, credit losses on securities owned, and interest rate risk are as follows: • Foreign currency interest rate swaps previously designated as hedges were used to hedge the currency risk of certain foreign currency-denominated fixed maturity securities owned for portfolio diversification. These derivatives were effective hedges prior to novation to a new counterparty. In conjunction with the novation, these derivatives were de-designated as hedges. We agree to pay, at specified intervals, fixed rate foreign currency-denominated principal and interest payments in exchange for fixed rate payments in the functional currency of the operating segment. We hold offsetting swaps wherein we agree to pay fixed rate principal and interest payments in the functional currency of the operating segment in exchange for fixed rate foreign currency-denominated payments. • Credit default swaps are used as economic hedges against credit risk but do not qualify for hedge accounting. A credit default swap is an agreement in which we agree with another party to pay, at specified intervals, a fixed-rate fee in exchange for insurance against a credit event on a specific investment. If a defined credit event occurs, our counterparty may either pay us a net cash settlement or we may surrender the specific investment to them in exchange for cash equal to the full notional amount of the swap. Credit events typically include events such as bankruptcy, failure to pay, or certain types of debt restructuring. • Interest rate swap was used to effectively convert certain of our floating rate, long-term debt into fixed rate long-term debt. Under this swap agreement, we received a variable rate of interest and paid a fixed rate of interest. Derivative Risks The basic types of risks associated with derivatives are market risk (that the value of the derivative will be adversely impacted by changes in the market, primarily the change in interest and exchange rates) and credit risk (that the counterparty will not perform according to the terms of the contract). The market risk of the derivatives should generally offset the market risk associated with the hedged financial instrument or liability. To help limit the credit exposure of the derivatives, we enter into master netting agreements with our counterparties whereby contracts in a gain position can be offset against contracts in a loss position. We also typically enter into bilateral, cross-collateralization agreements with our counterparties to help limit the credit exposure of the derivatives. These agreements require the counterparty in a loss position to submit acceptable collateral with the other counterparty in the event the net loss position meets or exceeds an agreed upon amount. Our credit exposure on derivatives, which is limited to the value of those contracts in a net gain position, including accrued interest receivable less collateral held, was $0.1 million at March 31, 2018 . The table below summarizes the nature and amount of collateral received from and posted to our derivative counterparties. March 31, 2018 December 31, 2017 (in millions of dollars) Carrying Value of Collateral Received from Counterparties Cash $ 16.2 $ 15.7 Carrying Value of Collateral Posted to Counterparties Fixed Maturity Securities $ 49.6 $ 46.4 See Note 4 for further discussion of our master netting agreements. The majority of our derivative instruments contain provisions that require us to maintain specified issuer credit ratings and financial strength ratings. Should our ratings fall below these specified levels, we would be in violation of the provisions, and our derivatives counterparties could terminate our contracts and request immediate payment. The aggregate fair value of all derivative instruments with credit risk-related contingent features that were in a liability position was $51.4 million and $52.2 million at March 31, 2018 and December 31, 2017 , respectively. Derivative Transactions The table below summarizes, by notional amounts, the activity for each category of derivatives. The notional amounts represent the basis upon which our counterparty pay and receive amounts are calculated. Swaps Receive Variable/Pay Fixed Receive Fixed/Pay Fixed Receive Fixed/Pay Variable Credit Default Forwards Total (in millions of dollars) Balance at December 31, 2016 $ 105.5 $ 616.5 $ 250.0 $ 70.0 $ 10.0 $ 1,052.0 Additions — — — — — — Terminations 0.1 20.3 — — 10.0 30.4 Balance at March 31, 2017 $ 105.4 $ 596.2 $ 250.0 $ 70.0 $ — $ 1,021.6 Balance at December 31, 2017 $ 48.0 $ 536.5 $ 250.0 $ 70.0 $ — $ 904.5 Additions — 19.9 — — 26.8 46.7 Terminations — — — 70.0 — 70.0 Balance at March 31, 2018 $ 48.0 $ 556.4 $ 250.0 $ — $ 26.8 $ 881.2 Cash Flow Hedges As of March 31, 2018 and December 31, 2017 , we had $343.9 million notional amount of receive fixed, pay fixed, open current and forward foreign currency interest rate swaps to hedge fixed income foreign currency-denominated securities. As of March 31, 2018 , we expect to amortize approximately $65.1 million of net deferred gains on derivative instruments during the next twelve months. This amount will be reclassified from accumulated other comprehensive income into earnings and reported on the same income statement line item as the hedged item. The income statement line items that will be affected by this amortization are net investment income and interest and debt expense. Additional amounts that may be reclassified from accumulated other comprehensive income into earnings to offset the earnings impact of foreign currency translation of hedged items are not estimable. As of March 31, 2018 , we are hedging the variability of future cash flows associated with forecasted transactions through the year 2038. Fair Value Hedges As of March 31, 2018 and December 31, 2017 , we had $48.0 million notional amount of receive variable, pay fixed interest rate swaps to hedge the changes in fair value of certain fixed rate securities held. These swaps effectively convert the associated fixed rate securities into floating rate securities, which are used to fund our floating rate long-term debt. The change in fair value of the hedged fixed maturity securities attributable to the hedged benchmark interest rate resulted in a loss of $0.5 million and $1.1 million for the three months ended March 31, 2018 and 2017, respectively, with an offsetting gain on the related interest rate swaps. As of March 31, 2018, we had $19.9 million notional amount of receive fixed, pay fixed, open current and forward foreign currency interest rate swaps to hedge fixed income foreign currency-denominated securities. The change in fair value of the hedged fixed maturity securities attributable to the change in the foreign currency exchange rate resulted in a loss of $0.3 million during the three months ended March 31, 2018 with an offsetting gain on the related forward foreign currency interest rate swaps. As of March 31, 2018 and December 31, 2017 , we had $250.0 million notional amount of receive fixed, pay variable interest rate swaps to hedge the changes in the fair value of certain fixed rate long-term debt. These swaps effectively convert the associated fixed rate long-term debt into floating rate debt and provide for a better matching of interest rates with our short-term investments, which have frequent interest rate resets similar to a floating rate security. The change in fair value of the hedged debt attributable to the hedged benchmark interest rate resulted in a gain of $2.4 million and $0.5 million for the three months ended March 31, 2018 and 2017, respectively, with an offsetting loss on the related interest rate swaps. The following table summarizes the carrying amount of hedged assets and liabilities and the related cumulative basis adjustments related to our fair value hedges. Carrying Amount of Hedged Assets (Liabilities) Cumulative Amount of Fair Value Hedging Adjustment Included in the Carrying Amount of the Hedged Assets (Liabilities) March 31, 2018 December 31, 2017 March 31, 2018 December 31, 2017 (in millions of dollars) Fixed maturity securities: Receive variable interest rate, pay fixed interest rate $ 48.1 $ 48.5 $ 0.1 $ 0.6 Receive fixed foreign currency interest, pay fixed foreign currency interest 19.4 — (0.3 ) — Long-term Debt (242.5 ) (244.8 ) 6.9 4.5 For the three months ended March 31, 2018 , a de minimis amount of the derivative instruments' gain or loss was excluded from the assessment of hedge effectiveness. For the three months ended March 31, 2017 , no component of the derivative instruments' gain or loss was excluded from the assessment of hedge effectiveness. There were no instances wherein we discontinued fair value hedge accounting due to a hedged firm commitment no longer qualifying as a fair value hedge. Derivatives not Designated as Hedging Instruments As of March 31, 2018 and December 31, 2017 , we held $192.6 million notional amount of receive fixed, pay fixed, foreign currency interest rate swaps. These derivatives are not designated as hedges, and as such, changes in fair value related to these derivatives are reported in earnings as a component of net realized investment gain or loss. As of December 31, 2017 , we held $70.0 million notional amount of single name credit default swaps. We entered into these swaps in order to mitigate the credit risk associated with specific securities owned. These swaps matured during the first quarter of 2018 and a de minimis gain was recognized on the maturities. We have an embedded derivative in a modified coinsurance arrangement for which we include in our realized investment gains and losses a calculation intended to estimate the value of the option of our reinsurance counterparty to cancel the reinsurance contract with us. However, neither party can unilaterally terminate the reinsurance agreement except in extreme circumstances resulting from regulatory supervision, delinquency proceedings, or other direct regulatory action. Cash settlements or collateral related to this embedded derivative are not required at any time during the reinsurance contract or at termination of the reinsurance contract. There are no credit-related counterparty triggers, and any accumulated embedded derivative gain or loss reduces to zero over time as the reinsured business winds down. Locations and Amounts of Derivative Financial Instruments The following tables summarize the location and fair values of derivative financial instruments, as reported in our consolidated balance sheets. March 31, 2018 Derivative Assets Derivative Liabilities Balance Sheet Location Fair Value Balance Sheet Location Fair Value (in millions of dollars) Designated as Hedging Instruments Cash Flow Hedges Forwards Other L-T Investments $ — Other Liabilities $ 0.2 Foreign Exchange Contracts Other L-T Investments 21.2 Other Liabilities 17.4 Total Cash Flow Hedges 21.2 17.6 Fair Value Hedges Interest Rate Swaps Other L-T Investments — Other Liabilities 7.0 Foreign Exchange Contracts Other L-T Investments 0.3 Other Liabilities — Total Fair Value Hedges 0.3 7.0 Total Designated as Hedging Instruments $ 21.5 $ 24.6 Not Designated as Hedging Instruments Foreign Exchange Contracts Other Liabilities $ 26.8 Embedded Derivative in Modified Coinsurance Arrangement Other Liabilities 17.6 Total Not Designated as Hedging Instruments $ 44.4 December 31, 2017 Derivative Assets Derivative Liabilities Balance Sheet Location Fair Value Balance Sheet Location Fair Value (in millions of dollars) Designated as Hedging Instruments Cash Flow Hedges Foreign Exchange Contracts Other L-T Investments $ 19.5 Other Liabilities $ 19.4 Total Cash Flow Hedges 19.5 19.4 Fair Value Hedges Interest Rate Swaps Other L-T Investments — Other Liabilities 5.1 Total Fair Value Hedges — 5.1 Total Designated as Hedging Instruments $ 19.5 $ 24.5 Not Designated as Hedging Instruments Credit Default Swaps Other Liabilities $ 0.2 Foreign Exchange Contracts Other Liabilities 27.5 Embedded Derivative in Modified Coinsurance Arrangement Other Liabilities 15.9 Total Not Designated as Hedging Instruments $ 43.6 The following table summarizes the location of gains and losses of derivative financial instruments designated as hedging instruments, as reported in our consolidated statements of income. Three Months Ended March 31 2018 2017 Net Investment Income Net Realized Investment Gain (Loss) Interest and Debt Expense Net Investment Income Net Realized Investment Gain (Loss) Interest and Debt Expense (in millions of dollars) Total Income and Expense Presented in the Consolidated Statements of Income of Which Hedged Items are Recorded $ 602.3 $ (2.2 ) $ 40.2 $ 602.4 $ 11.0 $ 39.8 Gain (Loss) on Cash Flow Hedging Relationships Interest Rate Swaps: Hedged items 77.8 0.1 11.4 81.9 0.2 11.4 Derivatives Designated as Hedging Instruments 15.9 (0.1 ) 0.6 14.6 — 0.5 Foreign Exchange Contracts Hedged items 5.0 — — 5.4 1.2 — Derivatives Designated as Hedging Instruments (0.3 ) — — (0.2 ) (1.2 ) — Gain (Loss) on Fair Value Hedging Relationships Interest Rate Swaps: Hedged items 0.7 1.9 3.6 1.4 (0.6 ) 3.6 Derivatives Designated as Hedging Instruments (0.4 ) (1.9 ) 0.1 (1.0 ) 0.6 (0.3 ) Foreign Exchange Contracts Hedged items 0.1 (0.3 ) — — — — Derivatives Designated as Hedging Instruments — 0.3 — — — — The following table summarizes the location of gains and losses of derivative financial instruments designated as cash flow hedging instruments, as reported in our consolidated statements of comprehensive income. Three Months Ended March 31 2018 2017 (in millions of dollars) Gain (Loss) Recognized in Other Comprehensive Income (Loss) on Derivatives Interest Rate Swaps and Forwards $ (0.2 ) $ — Foreign Exchange Contracts 3.8 (4.5 ) Total $ 3.6 $ (4.5 ) The following table summarizes the location of gains and losses on our derivatives not designated as hedging instruments, as reported in our consolidated statements of income. Three Months Ended March 31 2018 2017 (in millions of dollars) Net Realized Investment Gain (Loss) Credit Default Swaps $ — $ (0.3 ) Foreign Exchange Contracts 0.7 0.2 Embedded Derivative in Modified Coinsurance Arrangement (1.7 ) 8.6 Total $ (1.0 ) $ 8.5

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated Other Comprehensive Income (Loss)	Components of our accumulated other comprehensive income (loss), after tax, and related changes are as follows: Net Unrealized Gain on Securities Net Gain on Hedges Foreign Currency Translation Adjustment Unrecognized Pension and Postretirement Benefit Costs Total (in millions of dollars) Balance at December 31, 2017 $ 607.8 $ 282.3 $ (254.5 ) $ (508.1 ) $ 127.5 Adjustment to Adopt Accounting Standard Update - Note 2 (17.5 ) — — — (17.5 ) Balance at January 1, 2018 $ 590.3 $ 282.3 $ (254.5 ) $ (508.1 ) $ 110.0 Other Comprehensive Income (Loss) Before Reclassifications (234.8 ) 2.9 47.5 (1.2 ) (185.6 ) Amounts Reclassified from Accumulated Other Comprehensive Income or Loss 0.6 (11.7 ) — 4.3 (6.8 ) Net Other Comprehensive Income (Loss) (234.2 ) (8.8 ) 47.5 3.1 (192.4 ) Balance at March 31, 2018 $ 356.1 $ 273.5 $ (207.0 ) $ (505.0 ) $ (82.4 ) Balance at December 31, 2016 $ 440.6 $ 327.5 $ (354.0 ) $ (465.1 ) $ (51.0 ) Other Comprehensive Income (Loss) Before Reclassifications 44.7 (2.7 ) 17.1 (0.6 ) 58.5 Amounts Reclassified from Accumulated Other Comprehensive Income or Loss (2.3 ) (8.0 ) — 3.2 (7.1 ) Net Other Comprehensive Income (Loss) 42.4 (10.7 ) 17.1 2.6 51.4 Balance at March 31, 2017 $ 483.0 $ 316.8 $ (336.9 ) $ (462.5 ) $ 0.4 The net unrealized gain on securities consists of the following components: March 31 January 1 2018 2018 Change (in millions of dollars) Fixed Maturity Securities $ 4,348.2 $ 5,665.2 $ (1,317.0 ) Deferred Acquisition Costs (41.2 ) (51.4 ) 10.2 Reserves for Future Policy and Contract Benefits (4,030.1 ) (5,094.7 ) 1,064.6 Reinsurance Recoverable 321.0 375.8 (54.8 ) Income Tax (241.8 ) (304.6 ) 62.8 Total $ 356.1 $ 590.3 $ (234.2 ) March 31 December 31 2017 2016 Change (in millions of dollars) Fixed Maturity Securities $ 4,925.6 $ 4,664.6 $ 261.0 Other Investments (17.8 ) (22.7 ) 4.9 Deferred Acquisition Costs (40.9 ) (38.9 ) (2.0 ) Reserves for Future Policy and Contract Benefits (4,467.3 ) (4,253.2 ) (214.1 ) Reinsurance Recoverable 335.6 321.3 14.3 Income Tax (252.2 ) (230.5 ) (21.7 ) Total $ 483.0 $ 440.6 $ 42.4 Amounts reclassified from accumulated other comprehensive income or loss were recognized in our consolidated statements of income as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Net Unrealized Gain on Securities Net Realized Investment Gain (Loss) Net Gain on Sales of Securities and Other Invested Assets $ 0.2 $ 3.4 Other-Than-Temporary Impairment Loss (1.0 ) — (0.8 ) 3.4 Income Tax Expense (Benefit) (0.2 ) 1.1 Total $ (0.6 ) $ 2.3 Net Gain on Hedges Net Investment Income Gain on Interest Rate Swaps and Forwards $ 15.6 $ 14.3 Loss on Foreign Exchange Contracts (0.3 ) (0.3 ) Net Realized Investment Gain (Loss) Loss on Interest Rate Swaps (0.1 ) — Loss on Foreign Exchange Contracts — (1.2 ) Interest and Debt Expense Loss on Interest Rate Swaps (0.5 ) (0.5 ) 14.7 12.3 Income Tax Expense 3.0 4.3 Total $ 11.7 $ 8.0 Unrecognized Pension and Postretirement Benefit Costs Other Expenses Amortization of Net Actuarial Loss $ (5.6 ) $ (5.1 ) Amortization of Prior Service Credit 0.1 0.2 (5.5 ) (4.9 ) Income Tax Benefit (1.2 ) (1.7 ) Total $ (4.3 ) $ (3.2 )

Liability for Unpaid Claims and

Liability for Unpaid Claims and CAE Liability for Unpaid Claims	3 Months Ended
	Mar. 31, 2018
Insurance [Abstract]
Liability for Future Policy Benefits and Unpaid Claims Disclosure	Changes in the liability for unpaid claims and claim adjustment expenses are as follows: 2018 2017 (in millions of dollars) Balance at January 1 $ 23,222.0 $ 23,249.5 Less Reinsurance Recoverable 2,182.0 2,163.6 Net Balance at January 1 21,040.0 21,085.9 Incurred Related to Current Year 1,543.1 1,489.8 Prior Years Interest 283.6 295.7 All Other Incurred (192.0 ) (184.8 ) Foreign Currency 72.2 29.8 Total Incurred 1,706.9 1,630.5 Paid Related to Current Year (322.0 ) (283.9 ) Prior Years (1,400.4 ) (1,369.3 ) Total Paid (1,722.4 ) (1,653.2 ) Net Balance at March 31 21,024.5 21,063.2 Plus Reinsurance Recoverable 2,184.1 2,139.1 Balance at March 31 $ 23,208.6 $ 23,202.3 The majority of the net balances are related to disability claims with long-tail payouts on which interest earned on assets backing liabilities is an integral part of pricing and reserving. Interest accrued on prior year reserves has been calculated on the opening reserve balance less one-half of the period's claim payments relative to prior years at our average reserve discount rate for the respective periods. "Incurred Related to Prior Years - All Other Incurred" shown in the preceding chart is primarily impacted by the level of claim resolutions in the period relative to the long-term expectations reflected in the reserves. Our claim resolution rate assumption used in determining reserves is our expectation of the resolution rate we will experience over the life of the block of business and will vary from actual experience in any one period, both favorably and unfavorably. Reconciliation A reconciliation of policy and contract benefits and reserves for future policy and contract benefits as reported in our consolidated balance sheets to the liability for unpaid claims and claim adjustment expenses is as follows: March 31 2018 2017 (in millions of dollars) Policy and Contract Benefits $ 1,623.1 $ 1,544.8 Reserves for Future Policy and Contract Benefits 44,668.5 44,502.2 Total 46,291.6 46,047.0 Less: Life Reserves for Future Policy and Contract Benefits 8,278.7 8,105.7 Accident and Health Active Life Reserves 10,774.2 10,271.7 Adjustment Related to Unrealized Investment Gains and Losses 4,030.1 4,467.3 Liability for Unpaid Claims and Claim Adjustment Expenses $ 23,208.6 $ 23,202.3 The adjustment related to unrealized investment gains and losses reflects the changes that would be necessary to policyholder liabilities if the unrealized investment gains and losses related to the corresponding available-for-sale securities had been realized. Changes in this adjustment are reported as a component of other comprehensive income or loss.

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Segment Information	We have three principal operating business segments: Unum US, Unum UK, and Colonial Life. Our other segments are Closed Block and Corporate. Definitive Purchase Agreement In January of 2018, we entered into a definitive agreement to acquire Pramerica Zycie TUiR SA ("Pramerica"), a financial protection benefits provider in Poland. The acquisition of Pramerica will expand our European presence, which we believe to be an attractive market for financial protection benefits. The transaction is expected to close by the end of 2018 subject to customary approvals and closing conditions. Segment information is as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Premium Income Unum US Group Disability Group Long-term Disability $ 441.2 $ 434.4 Group Short-term Disability 176.3 157.9 Group Life and Accidental Death & Dismemberment Group Life 399.2 367.8 Accidental Death & Dismemberment 38.7 36.6 Supplemental and Voluntary Individual Disability 104.9 107.0 Voluntary Benefits 229.8 215.2 Dental and Vision 48.7 41.5 1,438.8 1,360.4 Unum UK Group Long-term Disability 90.8 80.8 Group Life 28.6 24.8 Supplemental 20.2 15.7 139.6 121.3 Colonial Life Accident, Sickness, and Disability 231.3 219.1 Life 81.0 74.2 Cancer and Critical Illness 86.0 81.0 398.3 374.3 Closed Block Individual Disability 109.4 121.3 Long-term Care 161.3 163.1 All Other 2.6 2.5 273.3 286.9 Total Premium Income $ 2,250.0 $ 2,142.9 Unum US Unum UK Colonial Life Closed Block Corporate Total (in millions of dollars) Three Months Ended March 31, 2018 Premium Income $ 1,438.8 $ 139.6 $ 398.3 $ 273.3 $ — $ 2,250.0 Net Investment Income 194.2 27.6 37.3 337.7 5.5 602.3 Other Income 29.0 — 0.3 19.0 1.2 49.5 Adjusted Operating Revenue $ 1,662.0 $ 167.2 $ 435.9 $ 630.0 $ 6.7 $ 2,901.8 Adjusted Operating Income (Loss) $ 243.9 $ 29.8 $ 81.0 $ 28.9 $ (40.3 ) $ 343.3 Three Months Ended March 31, 2017 Premium Income $ 1,360.4 $ 121.3 $ 374.3 $ 286.9 $ — $ 2,142.9 Net Investment Income 202.5 26.6 35.1 335.3 2.9 602.4 Other Income 28.7 — 0.3 20.8 0.4 50.2 Adjusted Operating Revenue $ 1,591.6 $ 147.9 $ 409.7 $ 643.0 $ 3.3 $ 2,795.5 Adjusted Operating Income (Loss) $ 239.1 $ 26.6 $ 82.4 $ 31.6 $ (39.8 ) $ 339.9 March 31 December 31 2018 2017 (in millions of dollars) Assets Unum US $ 18,036.3 $ 18,109.1 Unum UK 3,497.6 3,428.1 Colonial Life 4,292.9 4,184.1 Closed Block 34,603.8 35,051.2 Corporate 2,581.7 3,240.6 Total Assets $ 63,012.3 $ 64,013.1 We measure and analyze our segment performance on the basis of "adjusted operating revenue" and "adjusted operating income" or "adjusted operating loss", which differ from total revenue and income before income tax as presented in our consolidated statements of income due to the exclusion of net realized investment gains and losses and certain other items as specified in the reconciliations below. We believe adjusted operating revenue and adjusted operating income or loss are better performance measures and better indicators of the revenue and profitability and underlying trends in our business. These performance measures are in accordance with GAAP guidance for segment reporting, but they should not be viewed as a substitute for total revenue, income before income tax, or net income. Realized investment gains or losses depend on market conditions and do not necessarily relate to decisions regarding the underlying business of our segments. Our investment focus is on investment income to support our insurance liabilities as opposed to the generation of realized investment gains or losses. Although we may experience realized investment gains or losses which will affect future earnings levels, a long-term focus is necessary to maintain profitability over the life of the business since our underlying business is long-term in nature, and we need to earn the interest rates assumed in calculating our liabilities. We may at other times exclude certain other items from our discussion of financial ratios and metrics in order to enhance the understanding and comparability of our operational performance and the underlying fundamentals, but this exclusion is not an indication that similar items may not recur and does not replace net income or net loss as a measure of our overall profitability. See Note 11 for further discussion regarding the loss from a guaranty fund assessment. A reconciliation of total revenue to "adjusted operating revenue" and income before income tax to "adjusted operating income" is as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Total Revenue $ 2,899.6 $ 2,806.5 Excluding: Net Realized Investment Gain (Loss) (2.2 ) 11.0 Adjusted Operating Revenue $ 2,901.8 $ 2,795.5 Income Before Income Tax $ 341.1 $ 330.3 Excluding: Net Realized Investment Gain (Loss) (2.2 ) 11.0 Loss from Guaranty Fund Assessment — (20.6 ) Adjusted Operating Income $ 343.3 $ 339.9

Employee Benefit Plans

Employee Benefit Plans	3 Months Ended
	Mar. 31, 2018
Defined Benefit Plan [Abstract]
Employee Benefit Plans	Employee Benefit Plans Defined Benefit Pension and Other Postretirement Benefit (OPEB) Plans We sponsor several defined benefit pension and OPEB plans for our employees, including non-qualified pension plans. The U.S. qualified and non-qualified defined benefit pension plans comprise the majority of our total benefit obligation and benefit cost. We maintain a separate defined benefit plan for eligible employees in our U.K. operation. The U.S. defined benefit pension plans were closed to new entrants on December 31, 2013, the OPEB plan was closed to new entrants on December 31, 2012, and the U.K. plan was closed to new entrants on December 31, 2002. The following table provides the components of the net periodic benefit cost (credit) for the defined benefit pension and OPEB plans. Three Months Ended March 31 Pension Benefits U.S. Plans U.K. Plan OPEB 2018 2017 2018 2017 2018 2017 (in millions of dollars) Service Cost $ 2.3 $ 2.0 $ — $ — $ — $ — Interest Cost 19.9 21.1 1.5 1.5 1.2 1.5 Expected Return on Plan Assets (26.1 ) (25.8 ) (2.4 ) (2.0 ) (0.1 ) (0.2 ) Amortization of: Net Actuarial Loss 5.4 4.9 0.2 0.2 — — Prior Service Credit — (0.1 ) — — (0.1 ) (0.1 ) Total $ 1.5 $ 2.1 $ (0.7 ) $ (0.3 ) $ 1.0 $ 1.2 The service cost component of net periodic pension and postretirement benefit cost is included as a component of compensation expense in our consolidated statements of income. All other components of net periodic pension and postretirement benefit cost are included in other expenses.

Stockholders' Equity and Earnin

Stockholders' Equity and Earnings Per Common Share	3 Months Ended
	Mar. 31, 2018
Stockholders' Equity and Earnings Per Common Share [Abstract]
Earnings Per Share Disclosure	Earnings Per Common Share Net income per common share is determined as follows: Three Months Ended March 31 2018 2017 (in millions of dollars, except share data) Numerator Net Income $ 273.5 $ 229.9 Denominator (000s) Weighted Average Common Shares - Basic 221,894.0 229,429.6 Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards 683.0 949.2 Weighted Average Common Shares - Assuming Dilution 222,577.0 230,378.8 Net Income Per Common Share Basic $ 1.23 $ 1.00 Assuming Dilution $ 1.23 $ 1.00 We use the treasury stock method to account for the effect of outstanding stock options, nonvested restricted stock units, and nonvested performance share units on the computation of diluted earnings per share. Under this method, these potential common shares will each have a dilutive effect, as individually measured, when the average market price of Unum Group common stock during the period exceeds the exercise price of the stock options and the grant price of the nonvested restricted stock units and the nonvested performance share units. The outstanding stock options have exercise prices ranging from $23.35 to $26.29 , the nonvested restricted stock units have grant prices ranging from $27.85 to $55.26 , and the nonvested performance share units have grant prices ranging from $27.85 to $49.86 . In computing earnings per share assuming dilution, only potential common shares that are dilutive (those that reduce earnings per share) are included. Potential common shares excluded in the computation of diluted earnings per share because the impact would be antidilutive, based on then current market prices, approximated a de minimis amount and 0.4 million for the three months ended March 31, 2018 and 2017 , respectively.
Stockholders' Equity Disclosure	Common Stock During the second quarter of 2017, our board of directors authorized the repurchase of up to $750.0 million of Unum Group's outstanding common stock through November 25, 2018. This authorization replaced the previous authorization of $750.0 million that was scheduled to expire on November 26, 2017. The remaining repurchase amount under the new program was $412.8 million at March 31, 2018 . Common stock repurchases, which are accounted for using the cost method and classified as treasury stock until otherwise retired, were as follows: Three Months Ended March 31 2018 2017 (in millions) Shares Repurchased 1.9 2.1 Cost of Shares Repurchased 1 $ 100.2 $ 100.0 1 Includes commissions of $0.2 million for the three months ended March 31, 2018 , and a de minimis amount for the three months ended March 31, 2017 . Preferred Stock Unum Group has 25.0 million shares of preferred stock authorized with a par value of $0.10 per share. No preferred stock has been issued to date.

Commitments and Contingent Liab

Commitments and Contingent Liabilities	3 Months Ended
	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies Disclosure	Commitments and Contingent Liabilities Contingent Liabilities We are a defendant in a number of litigation matters. In some of these matters, no specified amount is sought. In others, very large or indeterminate amounts, including punitive and treble damages, are asserted. There is a wide variation of pleading practice permitted in the United States courts with respect to requests for monetary damages, including some courts in which no specified amount is required and others which allow the plaintiff to state only that the amount sought is sufficient to invoke the jurisdiction of that court. Further, some jurisdictions permit plaintiffs to allege damages well in excess of reasonably possible verdicts. Based on our extensive experience and that of others in the industry with respect to litigating or resolving claims through settlement over an extended period of time, we believe that the monetary damages asserted in a lawsuit or claim bear little relation to the merits of the case, or the likely disposition value. Therefore, the specific monetary relief sought is not stated. Unless indicated otherwise in the descriptions below, reserves have not been established for litigation and contingencies. An estimated loss is accrued when it is both probable that a liability has been incurred and the amount of the loss can be reasonably estimated. Claims Handling Matters We and our insurance subsidiaries, in the ordinary course of our business, are engaged in claim litigation where disputes arise as a result of a denial or termination of benefits. Most typically these lawsuits are filed on behalf of a single claimant or policyholder, and in some of these individual actions punitive damages are sought, such as claims alleging bad faith in the handling of insurance claims. For our general claim litigation, we maintain reserves based on experience to satisfy judgments and settlements in the normal course. We expect that the ultimate liability, if any, with respect to general claim litigation, after consideration of the reserves maintained, will not be material to our consolidated financial condition. Nevertheless, given the inherent unpredictability of litigation, it is possible that an adverse outcome in certain claim litigation involving punitive damages could, from time to time, have a material adverse effect on our consolidated results of operations in a period, depending on the results of operations for the particular period. From time to time class action allegations are pursued where the claimant or policyholder purports to represent a larger number of individuals who are similarly situated. Since each insurance claim is evaluated based on its own merits, there is rarely a single act or series of actions which can properly be addressed by a class action. Nevertheless, we monitor these cases closely and defend ourselves appropriately where these allegations are made. Miscellaneous Matters Similar to other insurers, we were recently the subject of an examination by a third party acting on behalf of a number of state treasurers concerning our compliance with the unclaimed property laws of the participating states. We cooperated fully with this examination and in the fourth quarter of 2017, we started the process to reach a Global Resolution Agreement with the third party regarding settlement of the examination, which we finalized in January of 2018. Under the terms of the agreement, the third party acting on behalf of the signatory states compared insured data to the Social Security Administration's Death Master File (SSDMF) to identify deceased insureds and contract holders where a valid claim has not been made. During the fourth quarter of 2017, we established reserves which reflect our estimate of the liability expected to be paid as we execute on the terms of the settlement. We also are cooperating with a Delaware Market Conduct examination involving the same issue, which is currently inactive, as well as defending litigation on this issue in West Virginia, as described in the following paragraph. The legal and regulatory environment around unclaimed death benefits continues to evolve. It is possible that the current settlement and/or similar investigations by other state jurisdictions may result in payments to beneficiaries, the payment of abandoned funds under state law, and/or administrative penalties, the total of which may be in excess of the reserves established. In December 2012, State of West Virginia ex rel. John D. Perdue v. Provident Life and Accident Insurance Company and State of West Virginia ex rel. John D. Perdue v. Colonial Life & Accident Insurance Company were filed in the Circuit Court of Putnam County, West Virginia. These two separate complaints alleged violations of the West Virginia Uniform Unclaimed Property Act by failing to identify and report all unclaimed insurance policy proceeds due to be escheated to West Virginia. The complaints sought to examine company records and assess penalties and costs in an undetermined amount. In December 2013, the court dismissed both complaints, holding that the West Virginia Uniform Unclaimed Property Act does not require insurance companies to periodically search the SSDMF or escheat unclaimed life insurance benefits until a claim has been submitted. In January 2014, the plaintiff appealed the dismissal of both complaints. In June 2015, the appellate court reinstated the case, holding that the West Virginia Uniform Unclaimed Property Act requires insurers to make reasonable efforts to determine whether their insureds are still living. The case was remanded to the trial court where we answered the complaints. In 2016, the West Virginia Legislature enacted a law defining insurers’ duties with regard to unclaimed benefits for life insurance policies, annuity contracts, and retained asset accounts issued in West Virginia. In December 2016, we filed a motion to dismiss the complaints in light of this law. In June 2017, the court denied the motion. The case is proceeding through the discovery phase. In 2009, a Pennsylvania-based insurance company and its affiliates were ordered into rehabilitation, and the Pennsylvania Insurance Commissioner, who was appointed as the Rehabilitator, filed petitions for liquidation with the Commonwealth Court of Pennsylvania. Under Pennsylvania law, payment of covered claims and other related insurance obligations are provided, within prescribed limits, by state guaranty associations. These guaranty associations assess fees to meet these obligations on insurance companies that sell insurance within the state, which are generally based on a company's pro rata portion of average premiums written or received for several years prior to the insolvency. In March 2017, a formal order of liquidation was issued, and as such, we were subject to an assessment by those guaranty associations that are responsible for policyholder claims, and accordingly accrued, in the first quarter of 2017, an estimated loss contingency. We continue to submit payment to satisfy this assessment as requests for payment are received from the guaranty associations. Summary Various lawsuits against us, in addition to those discussed above, have arisen in the normal course of business. Further, state insurance regulatory authorities and other federal and state authorities regularly make inquiries and conduct investigations concerning our compliance with applicable insurance and other laws and regulations. Given the complexity and scope of our litigation and regulatory matters, it is not possible to predict the ultimate outcome of all pending investigations or legal proceedings or provide reasonable estimates of potential losses, except if noted in connection with specific matters. It is possible that our results of operations or cash flows in a particular period could be materially affected by an ultimate unfavorable outcome of pending litigation or regulatory matters depending, in part, on our results of operations or cash flows for the particular period. We believe, however, that the ultimate outcome of all pending litigation and regulatory matters, after consideration of applicable reserves and rights to indemnification, should not have a material adverse effect on our financial position.

Debt

Debt	3 Months Ended
	Mar. 31, 2018
Debt and Other Disclosures [Abstract]
Debt Disclosure	Other Debt During the three months ended March 31, 2018 , we made principal payments of $15.0 million on our senior secured non-recourse notes issued by Northwind Holdings, LLC. At March 31, 2018 , letters of credit totaling $2.1 million had been issued from the credit facility, but there were no borrowed amounts outstanding.
Income Tax Disclosure	Income Tax On December 22, 2017, the U.S. Federal government enacted the TCJA, which reduces the federal corporate tax rate from 35 percent to 21 percent effective January 1, 2018. We are applying the guidance in Staff Accounting Bulletin (SAB) 118 when accounting for the enactment-date effects of TCJA. At March 31, 2018, our estimates recorded at December 31, 2017 for the tax effects of TCJA are not final. We will continue to refine our calculations as additional analysis is completed and record the final amounts during the one year measurement period after the enactment date as allowed by SAB 118. Tax rate estimates recorded at December 31, 2017 and March 31, 2018 may be impacted by changes in accounting and tax interpretations of the TCJA legislation.

Basis of Presentation (Policies

Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Basis of Accounting	The accompanying consolidated financial statements of Unum Group and its subsidiaries (the Company) have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. For further information, refer to the consolidated financial statements and footnotes included in our annual report on Form 10-K for the year ended December 31, 2017 .

Accounting Developments Account

Accounting Developments Accounting Developments (Tables)	3 Months Ended
	Mar. 31, 2018
Accounting Changes and Error Corrections [Abstract]
Accounting Updates Adopted in 2018	Summary of Financial Statement Impacts of Accounting Updates Adopted in 2018: For the Three Months Ended March 31, 2017 Historical Accounting Method As Adjusted Effect of Change (in millions of dollars) Adjustments due to ASC 230 Consolidated Statements of Cash Flow Cash Flows from Operating Activities Other, Net $ 2.2 $ 4.4 $ 2.2 Cash Flows from Investing Activities Proceeds from Sales and Maturities of Other Investments 48.7 46.5 (2.2 ) Adjustments due to ASC 715 Consolidated Statements of Income Compensation Expense 207.0 209.0 2.0 Other Expenses 230.8 228.8 (2.0 ) Balance at December 31, 2017 Balance at January 1, 2018 Effect of Change (in millions of dollars) Adjustments due to ASC 825 Consolidated Balance Sheets Assets Other Long-term Investments $ 646.8 $ 643.0 $ (3.8 ) Liabilities Deferred Income Tax 199.0 198.2 (0.8 ) Stockholders' Equity Accumulated Other Comprehensive Income (Loss) 127.5 110.0 (17.5 ) Retained Earnings 9,542.2 9,556.7 14.5

Fair Values of Financial Inst24

Fair Values of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Private Equity Partnerships	Private Equity Partnerships Our private equity partnerships represent funds that are primarily invested in private credit, private equity and real assets, as described below. Distributions received from the funds arise from income generated by the underlying investments as well as the liquidation of the underlying investments. There is generally not a public market for these investments. The following table presents additional information about our private equity partnerships as of March 31, 2018, including commitments for additional investments which may or may not be funded: Investment Category Fair Value Redemption Term / Redemption Notice Unfunded Commitments (in millions of dollars) (in millions of dollars) Private Credit (a) $ 165.5 Not redeemable $ 116.1 22.4 Initial 2 year lock on each new investment / Quarterly after 2 year lock with 90 days notice 2.0 Total Private Credit 187.9 118.1 Private Equity (b) 101.3 Not redeemable 106.2 Real Assets (c) 93.9 Not redeemable 83.0 30.2 Quarterly / 90 days notice — Total Real Assets 124.1 83.0 Total Partnerships $ 413.3 $ 307.3 (a) Private Credit - The limited partnerships described in this category employ various investment strategies, generally providing direct lending or other forms of debt financing including first-lien, second-lien, mezzanine, and subordinated loans. The limited partnerships have credit exposure to corporates, physical assets, and/or financial assets within variety of industries (including manufacturing, healthcare, energy, business services, technology, materials, and retail) in North America and, to a lesser extent, outside of North America. Unless specifically disclosed in the table above, these limited partnerships do not allow for redemptions. As of March 31, 2018 , the estimated remaining life of the investments that do not allow for redemptions is approximately 30 percent in the next 3 years, 42 percent during the period from 3 to 5 years, 23 percent during the period from 5 to 10 years, and 5 percent during the period from 10 to 15 years. (b) Private Equity - The limited partnerships described in this category employ various strategies generally investing in controlling or minority control equity positions directly in companies and/or assets across various industries (including manufacturing, healthcare, energy, business services, technology, materials, and retail), primarily in private markets within North America and, to a lesser extent, outside of North America. Unless specifically disclosed in the table above, these limited partnerships do not allow for redemptions. As of March 31, 2018 , the estimated remaining life of the investments that do not allow for redemptions is approximately 39 percent in the next 3 years, 31 percent during the period from 3 to 5 years, 28 percent during the period from 5 to 10 years, and 2 percent during the period from 10 to 15 years. (c) Real Assets - The limited partnerships described in this category employ various strategies, which include investing in the equity and/or debt financing of physical assets, including infrastructure (energy, power, water/wastewater, communications), transportation (including airports, ports, toll roads, aircraft, railcars) and real estate in North America, Europe, South America, and Asia. Unless specifically disclosed in the table above, these limited partnerships do not allow for redemption. As of March 31, 2018 , the estimated remaining life of the investments that do not allow for redemptions is approximately 3 percent in the next 3 years, 95 percent during the period from 5 to 10 years, and 2 percent during the period from 10 to 15 years.
Fair Value Measurements, Recurring	The following tables present information about assets and liabilities measured at fair value on a recurring basis by fair value level, based on the observability of the inputs used: March 31, 2018 Level 1 Level 2 Level 3 NAV Total (in millions of dollars) Assets Fixed Maturity Securities United States Government and Government Agencies and Authorities $ 495.3 $ 1,051.0 $ — $ — $ 1,546.3 States, Municipalities, and Political Subdivisions — 2,321.6 35.7 — 2,357.3 Foreign Governments — 841.4 32.3 — 873.7 Public Utilities 574.1 7,123.4 284.2 — 7,981.7 Mortgage/Asset-Backed Securities — 1,842.3 0.5 — 1,842.8 All Other Corporate Bonds 8,127.2 20,742.6 987.2 — 29,857.0 Redeemable Preferred Stocks — 19.1 22.2 — 41.3 Total Fixed Maturity Securities 9,196.6 33,941.4 1,362.1 — 44,500.1 Other Long-term Investments Derivatives Foreign Exchange Contracts — 21.5 — — 21.5 Equity Securities 24.9 46.4 1.1 — 72.4 Private Equity Partnerships — — — 413.3 413.3 Total Other Long-term Investments 24.9 67.9 1.1 413.3 507.2 Total Recurring Fair Value Measurements Assets $ 9,221.5 $ 34,009.3 $ 1,363.2 $ 413.3 $ 45,007.3 Liabilities Other Liabilities Derivatives Interest Rate Swaps $ — $ 7.2 $ — $ — $ 7.2 Foreign Exchange Contracts — 44.2 — — 44.2 Embedded Derivative in Modified Coinsurance Arrangement — — 17.6 — 17.6 Total Derivatives — 51.4 17.6 — 69.0 Total Recurring Fair Value Measurements Liabilities $ — $ 51.4 $ 17.6 $ — $ 69.0 December 31, 2017 Level 1 Level 2 Level 3 NAV Total (in millions of dollars) Assets Fixed Maturity Securities United States Government and Government Agencies and Authorities $ 460.1 $ 1,022.4 $ — $ — $ 1,482.5 States, Municipalities, and Political Subdivisions — 2,336.9 — — 2,336.9 Foreign Governments — 863.9 — — 863.9 Public Utilities 154.2 7,874.6 207.7 — 8,236.5 Mortgage/Asset-Backed Securities — 1,973.6 — — 1,973.6 All Other Corporate Bonds 3,556.1 25,816.2 1,150.1 — 30,522.4 Redeemable Preferred Stocks — 19.2 22.8 — 42.0 Total Fixed Maturity Securities 4,170.4 39,906.8 1,380.6 — 45,457.8 Other Long-term Investments Derivatives Foreign Exchange Contracts — 19.5 — — 19.5 Equity Securities 0.2 10.4 1.1 — 11.7 Private Equity Partnerships — — — 407.2 407.2 Total Other Long-term Investments 0.2 29.9 1.1 407.2 438.4 Total Recurring Fair Value Measurements Assets $ 4,170.6 $ 39,936.7 $ 1,381.7 $ 407.2 $ 45,896.2 Liabilities Other Liabilities Derivatives Interest Rate Swaps $ — $ 5.1 $ — $ — $ 5.1 Foreign Exchange Contracts — 46.9 — — 46.9 Credit Default Swaps — 0.2 — — 0.2 Embedded Derivative in Modified Coinsurance Arrangement — — 15.9 — 15.9 Total Derivatives — 52.2 15.9 — 68.1 Total Recurring Fair Value Measurements Liabilities $ — $ 52.2 $ 15.9 $ — $ 68.1
Transfers of Assets between Level 1 and Level 2	Transfers of assets between Level 1 and Level 2 are as follows: Three Months Ended March 31 2018 2017 Transfers into Level 1 from Level 2 from Level 1 from Level 2 from (in millions of dollars) Fixed Maturity Securities United States Government and Government Agencies and Authorities $ 40.1 $ — $ 54.5 $ 180.0 Public Utilities 528.5 62.0 1,103.9 57.5 All Other Corporate Bonds 5,274.9 953.3 6,317.0 1,075.3 Total Fixed Maturity Securities $ 5,843.5 $ 1,015.3 $ 7,475.4 $ 1,312.8 Equity Securities $ 24.2 $ 14.9 $ — $ — Transfers between Level 1 and Level 2 occurred due to the change in availability of either a TRACE or broker market maker price. Depending on current market conditions, the availability of these Level 1 prices can vary from period to period. For fair value measurements of financial instruments that were transferred either into or out of Level 1 or 2, we reflect the transfers using the fair value at the beginning of the period.
Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs	Changes in assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) are as follows: Three Months Ended March 31, 2018 Total Realized and Unrealized Investment Gains (Losses) Included in Level 3 Transfers Fair Value Beginning of Period Earnings Other Comprehensive Income or Loss Purchases Sales Into Out of Fair Value End of Period (in millions of dollars) Fixed Maturity Securities States, Municipalities, and Political Subdivisions $ — $ — $ (0.8 ) $ — $ (0.1 ) $ 36.6 $ — $ 35.7 Foreign Governments — — (0.4 ) — — 32.7 — 32.3 Public Utilities 207.7 — (6.7 ) — (1.1 ) 200.9 (116.6 ) 284.2 Mortgage/Asset-Backed Securities — — — — — 0.5 — 0.5 All Other Corporate Bonds 1,150.1 — (22.1 ) — (11.7 ) 466.0 (595.1 ) 987.2 Redeemable Preferred Stocks 22.8 — (0.6 ) — — — — 22.2 Total Fixed Maturity Securities 1,380.6 — (30.6 ) — (12.9 ) 736.7 (711.7 ) 1,362.1 Equity Securities 1.1 — — — — — — 1.1 Embedded Derivative in Modified Coinsurance Arrangement (15.9 ) (1.7 ) — — — — — (17.6 ) Three Months Ended March 31, 2017 Total Realized and Unrealized Investment Gains (Losses) Included in Level 3 Transfers Fair Value Beginning of Period Earnings Other Comprehensive Income or Loss Purchases Sales Into Out of Fair Value End of Period (in millions of dollars) Fixed Maturity Securities States, Municipalities, and Political Subdivisions $ 89.5 $ — $ — $ — $ — $ 0.4 $ (89.5 ) $ 0.4 Public Utilities 265.3 — 1.8 8.0 — 112.6 (75.2 ) 312.5 All Other Corporate Bonds 1,459.7 — 13.1 — (36.4 ) 143.9 (657.3 ) 923.0 Redeemable Preferred Stocks 23.2 — (0.3 ) — — — — 22.9 Total Fixed Maturity Securities 1,837.7 — 14.6 8.0 (36.4 ) 256.9 (822.0 ) 1,258.8 Equity Securities 1.2 — — — — — — 1.2 Embedded Derivative in Modified Coinsurance Arrangement (46.7 ) 8.6 — — — — — (38.1 ) Realized and unrealized investment gains and losses presented in the preceding tables represent gains and losses only for the time during which the applicable financial instruments were classified as Level 3. The transfers between levels resulted primarily from a change in observability of three inputs used to determine fair values of the securities transferred: (1) transactional data for new issuance and secondary trades, (2) broker/dealer quotes and pricing, primarily related to changes in the level of activity in the market and whether the market was considered orderly, and (3) comparable bond metrics from which to perform an analysis. For fair value measurements of financial instruments that were transferred either into or out of Level 3, we reflect the transfers using the fair value at the beginning of the period. We believe this allows for greater transparency, as all changes in fair value that arise during the reporting period of the transfer are disclosed as a component of our Level 3 reconciliation. Gains (losses) which are included in earnings and are attributable to the change in fair value of assets or liabilities valued using significant unobservable inputs and still held at period end were $(1.7) million and $8.6 million for the three months ended March 31, 2018 and 2017 , respectively. These amounts relate entirely to the change in fair value of an embedded derivative in a modified coinsurance arrangement and are reported as a component of realized investment gains and losses.
Quantitative Information Regarding Significant Unobservable Inputs	The table below provides quantitative information regarding the significant unobservable inputs used in Level 3 fair value measurements derived from internal models. Certain securities classified as Level 3 are excluded from the table below due to limitations in our ability to obtain the underlying inputs used by external pricing sources. March 31, 2018 Fair Value Valuation Method Unobservable Input Range/Weighted Average (in millions of dollars) Fixed Maturity Securities All Other Corporate Bonds - Private $ 276.6 Market Approach Comparability Adjustment (a) 1.00% - 1.25% / 1.04% Equity Securities - Private $ 1.1 Market Approach Market Convention (d) Priced at Cost or Owner's Equity Embedded Derivative in Modified Coinsurance Arrangement $ (17.6 ) Discounted Cash Flows Projected Liability Cash Flows (e) Actuarial Assumptions December 31, 2017 Fair Value Valuation Method Unobservable Input Range/Weighted Average (in millions of dollars) Fixed Maturity Securities All Other Corporate Bonds - Private $ 244.4 Market Approach Comparability Adjustment (a) 0.20% - 0.20% / 0.20% Equity Securities - Private $ 1.1 Market Approach Market Convention (d) Priced at Cost or Owner's Equity Embedded Derivative in Modified Coinsurance Arrangement $ (15.9 ) Discounted Cash Flows Projected Liability Cash Flows (e) Actuarial Assumptions (a) Represents basis point adjustments for changes in benchmark spreads associated with various industry sectors (b) Represents basis point adjustments to apply a discount due to the illiquidity of an investment (c) Represents basis point adjustments for credit-specific factors (d) Represents a decision to price based on par value, cost, or owner's equity when limited data is available (e) Represents various actuarial assumptions required to derive the liability cash flows including incidence, termination, and lapse rates
Fair Value, by Balance Sheet Grouping	The following table presents the carrying amounts and estimated fair values of our financial instruments not measured at fair value and indicates the level in the fair value hierarchy of the estimated fair value measurement based on the observability of the inputs used: March 31, 2018 Estimated Fair Value Level 1 Level 2 Level 3 Total Carrying Value (in millions of dollars) Assets Mortgage Loans $ — $ 2,235.1 $ — $ 2,235.1 $ 2,200.8 Policy Loans — — 3,666.0 3,666.0 3,573.3 Other Long-term Investments Miscellaneous Long-term Investments — 34.1 119.1 153.2 153.2 Total Financial Instrument Assets Not Carried at Fair Value $ — $ 2,269.2 $ 3,785.1 $ 6,054.3 $ 5,927.3 Liabilities Long-term Debt $ 2,155.7 $ 805.9 $ — $ 2,961.6 $ 2,721.9 Payables for Collateral on Investments Federal Home Loan Bank (FHLB) Funding Agreements — 350.0 — 350.0 350.0 Other Liabilities Unfunded Commitments — 3.7 — 3.7 3.7 Total Financial Instrument Liabilities Not Carried at Fair Value $ 2,155.7 $ 1,159.6 $ — $ 3,315.3 $ 3,075.6 December 31, 2017 Estimated Fair Value Level 1 Level 2 Level 3 Total Carrying Value (in millions of dollars) Assets Mortgage Loans $ — $ 2,306.2 $ — $ 2,306.2 $ 2,213.2 Policy Loans — — 3,677.5 3,677.5 3,571.1 Other Long-term Investments Miscellaneous Long-term Investments — 34.1 128.2 162.3 162.3 Total Financial Instrument Assets Not Carried at Fair Value $ — $ 2,340.3 $ 3,805.7 $ 6,146 $ 5,946.6 Liabilities Long-term Debt $ 1,171.8 $ 1,876.9 $ — $ 3,048.7 $ 2,738.4 Payables for Collateral on Investments Federal Home Loan Bank (FHLB) Funding Agreements — 350.0 — 350.0 350.0 Other Liabilities Unfunded Commitments — 3.7 — 3.7 3.7 Total Financial Instrument Liabilities Not Carried at Fair Value $ 1,171.8 $ 2,230.6 $ — $ 3,402.4 $ 3,092.1

Investments (Tables)

Investments (Tables)	3 Months Ended
	Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Amortized Cost and Fair Values of Securities by Security Type	At March 31, 2018 and December 31, 2017 , all fixed maturity securities were classified as available-for-sale. The amortized cost and fair values of securities by security type are shown as follows: March 31, 2018 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value (in millions of dollars) United States Government and Government Agencies and Authorities $ 1,411.0 $ 146.4 $ 11.1 $ 1,546.3 States, Municipalities, and Political Subdivisions 2,016.6 343.2 2.5 2,357.3 Foreign Governments 692.6 181.9 0.8 873.7 Public Utilities 6,919.6 1,089.7 27.6 7,981.7 Mortgage/Asset-Backed Securities 1,776.4 81.1 14.7 1,842.8 All Other Corporate Bonds 27,296.7 2,796.7 236.4 29,857.0 Redeemable Preferred Stocks 39.0 2.3 — 41.3 Total Fixed Maturity Securities $ 40,151.9 $ 4,641.3 $ 293.1 $ 44,500.1 December 31, 2017 Amortized Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value (in millions of dollars) United States Government and Government Agencies and Authorities $ 1,311.1 $ 176.1 $ 4.7 $ 1,482.5 States, Municipalities, and Political Subdivisions 1,942.8 395.4 1.3 2,336.9 Foreign Governments 673.0 191.3 0.4 863.9 Public Utilities 6,952.7 1,296.4 12.6 8,236.5 Mortgage/Asset-Backed Securities 1,873.2 105.1 4.7 1,973.6 All Other Corporate Bonds 26,988.7 3,633.5 99.8 30,522.4 Redeemable Preferred Stocks 39.0 3.0 — 42.0 Total Fixed Maturity Securities $ 39,780.5 $ 5,800.8 $ 123.5 $ 45,457.8
Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position	The following charts indicate the length of time our fixed maturity securities have been in a gross unrealized loss position. March 31, 2018 Less Than 12 Months 12 Months or Greater Fair Value Gross Unrealized Loss Fair Value Gross Unrealized Loss (in millions of dollars) United States Government and Government Agencies and Authorities $ 178.4 $ 8.0 $ 57.5 $ 3.1 States, Municipalities, and Political Subdivisions 134.3 1.4 24.2 1.1 Foreign Governments 38.8 0.8 — — Public Utilities 400.7 17.8 152.2 9.8 Mortgage/Asset-Backed Securities 366.3 9.4 114.1 5.3 All Other Corporate Bonds 5,376.8 149.7 745.2 86.7 Total Fixed Maturity Securities $ 6,495.3 $ 187.1 $ 1,093.2 $ 106.0 December 31, 2017 Less Than 12 Months 12 Months or Greater Fair Value Gross Unrealized Loss Fair Value Gross Unrealized Loss (in millions of dollars) United States Government and Government Agencies and Authorities $ 157.9 $ 3.0 $ 58.8 $ 1.8 States, Municipalities, and Political Subdivisions 45.7 0.5 22.3 0.7 Foreign Governments 13.2 0.4 — — Public Utilities 213.2 7.9 133.5 4.7 Mortgage/Asset-Backed Securities 252.5 1.4 144.7 3.3 All Other Corporate Bonds 1,355.1 26.8 785.2 73.0 Total Fixed Maturity Securities $ 2,037.6 $ 40.0 $ 1,144.5 $ 83.5
Distribution of the Maturity Dates for Fixed Maturity Securities	The following is a distribution of the maturity dates for fixed maturity securities. The maturity dates have not been adjusted for possible calls or prepayments. March 31, 2018 Total Amortized Cost Unrealized Gain Position Unrealized Loss Position Gross Gain Fair Value Gross Loss Fair Value (in millions of dollars) 1 year or less $ 1,694.9 $ 23.4 $ 1,645.0 $ 4.1 $ 69.2 Over 1 year through 5 years 5,665.0 372.0 5,680.4 22.3 334.3 Over 5 years through 10 years 12,273.8 903.8 9,124.4 138.1 3,915.1 Over 10 years 18,741.8 3,261.0 19,099.4 113.9 2,789.5 38,375.5 4,560.2 35,549.2 278.4 7,108.1 Mortgage/Asset-Backed Securities 1,776.4 81.1 1,362.4 14.7 480.4 Total Fixed Maturity Securities $ 40,151.9 $ 4,641.3 $ 36,911.6 $ 293.1 $ 7,588.5 December 31, 2017 Total Amortized Cost Unrealized Gain Position Unrealized Loss Position Gross Gain Fair Value Gross Loss Fair Value (in millions of dollars) 1 year or less $ 1,625.1 $ 30.9 $ 1,638.8 $ 3.1 $ 14.1 Over 1 year through 5 years 5,579.9 453.6 5,810.9 18.5 204.1 Over 5 years through 10 years 12,091.1 1,169.8 11,916.5 53.2 1,291.2 Over 10 years 18,611.2 4,041.4 21,333.1 44.0 1,275.5 37,907.3 5,695.7 40,699.3 118.8 2,784.9 Mortgage/Asset-Backed Securities 1,873.2 105.1 1,576.4 4.7 397.2 Total Fixed Maturity Securities $ 39,780.5 $ 5,800.8 $ 42,275.7 $ 123.5 $ 3,182.1
Available for Sale Debt Securities by External Credit Rating	The following chart depicts an analysis of our fixed maturity security portfolio between investment-grade and below-investment-grade categories as of March 31, 2018: Gross Unrealized Loss Fair Value Gross Unrealized Gain Amount Percent of Total Gross Unrealized Loss (in millions of dollars) Investment-Grade $ 41,233.8 $ 4,527.8 $ 198.7 67.8 % Below-Investment-Grade 3,266.3 113.5 94.4 32.2 Total Fixed Maturity Securities $ 44,500.1 $ 4,641.3 $ 293.1 100.0 %
Low Income Housing Tax Credits	The Company invests in tax credit partnerships primarily for the receipt of income tax credits and tax benefits derived from passive losses on the investments. Amounts recognized in the consolidated statements of income are as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Income Tax Credits $ 10.4 $ 10.4 Amortization, net of tax (7.0 ) (5.8 ) Income Tax Benefit $ 3.4 $ 4.6
Mortgage Loans by Property Type and Geographic Region	Mortgage loans by property type and geographic region are presented below. March 31, 2018 December 31, 2017 (in millions of dollars) Carrying Percent of Carrying Percent of Amount Total Amount Total Property Type Apartment $ 380.0 17.3 % $ 360.0 16.3 % Industrial 584.9 26.6 601.2 27.2 Office 674.5 30.6 692.3 31.3 Retail 529.5 24.1 527.6 23.8 Other 31.9 1.4 32.1 1.4 Total $ 2,200.8 100.0 % $ 2,213.2 100.0 % Region New England $ 55.2 2.5 % $ 56.1 2.5 % Mid-Atlantic 154.5 7.0 155.5 7.0 East North Central 291.3 13.2 282.0 12.8 West North Central 208.4 9.5 210.1 9.5 South Atlantic 481.7 21.9 494.4 22.3 East South Central 88.1 4.0 88.8 4.0 West South Central 245.4 11.2 247.4 11.2 Mountain 254.9 11.6 251.2 11.4 Pacific 421.3 19.1 427.7 19.3 Total $ 2,200.8 100.0 % $ 2,213.2 100.0 %
Mortgage Loans by Credit Quality Indicators	Mortgage loans, sorted by the applicable credit quality indicators, are as follows: March 31, 2018 December 31, 2017 (in millions of dollars) Internal Rating Aa $ 0.3 $ 0.4 A 481.9 445.7 Baa 1,704.4 1,753.0 Ba 14.2 14.1 Total $ 2,200.8 $ 2,213.2 Loan-to-Value Ratio <= 65% $ 1,082.6 $ 1,101.7 > 65% <= 75% 1,053.8 1,041.6 > 75% <= 85% 40.2 49.3 > 85% 24.2 20.6 Total $ 2,200.8 $ 2,213.2
Remaining Contractual Maturity of Securities Lending Agreements	The remaining contractual maturities of our securities lending agreements disaggregated by class of collateral pledged are as follows: March 31, 2018 December 31, 2017 Overnight and Continuous (in millions of dollars) United States Government and Government Agencies and Authorities $ 1.0 $ 0.2 Public Utilities — 0.5 All Other Corporate Bonds 15.0 29.8 Total Borrowings 16.0 30.5 Gross Amount of Recognized Liability for Securities Lending Transactions 16.0 30.5 Amounts Related to Agreements Not Included in Offsetting Disclosure Contained Herein $ — $ —
Federal Home Loan Bank Common Stock Carrying Amounts, Amounts Posted, and Advances Received	The carrying value of common stock owned, collateral posted, and advances received are as follows: March 31, 2018 December 31, 2017 (in millions of dollars) Carrying Value of FHLB Common Stock $ 34.1 $ 34.1 Advances from FHLB 350.0 350.0 Carrying Value of Collateral Posted to FHLB Fixed Maturity Securities $ 268.5 $ 213.3 Commercial Mortgage Loans 321.6 331.5 Total Carrying Value of Collateral Posted to FHLB $ 590.1 $ 544.8
Schedule of Financial Instrument and Derivative Offsetting	Shown below are our financial instruments that either meet the accounting requirements that allow them to be offset in our balance sheets or that are subject to an enforceable master netting arrangement or similar agreement. Our accounting policy is to not offset these financial instruments in our balance sheets. Net amounts disclosed below have been reduced by the amount of collateral pledged to or received from our counterparties. March 31, 2018 Gross Amount Gross Amount Not of Recognized Gross Amount Net Amount Offset in Balance Sheet Financial Offset in Presented in Financial Cash Net Instruments Balance Sheet Balance Sheet Instruments Collateral Amount (in millions of dollars) Financial Assets: Derivatives $ 21.5 $ — $ 21.5 $ (5.4 ) $ (16.1 ) $ — Securities Lending 104.2 — 104.2 (88.2 ) (16.0 ) — Total $ 125.7 $ — $ 125.7 $ (93.6 ) $ (32.1 ) $ — Financial Liabilities: Derivatives $ 51.4 $ — $ 51.4 $ (39.0 ) $ — $ 12.4 Securities Lending 16.0 — 16.0 (16.0 ) — — Total $ 67.4 $ — $ 67.4 $ (55.0 ) $ — $ 12.4 December 31, 2017 Gross Amount Gross Amount Not of Recognized Gross Amount Net Amount Offset in Balance Sheet Financial Offset in Presented in Financial Cash Net Instruments Balance Sheet Balance Sheet Instruments Collateral Amount (in millions of dollars) Financial Assets: Derivatives $ 19.5 $ — $ 19.5 $ (4.2 ) $ (15.3 ) $ — Securities Lending 159.2 — 159.2 (128.7 ) (30.5 ) — Total $ 178.7 $ — $ 178.7 $ (132.9 ) $ (45.8 ) $ — Financial Liabilities: Derivatives $ 52.2 $ — $ 52.2 $ (42.9 ) $ — $ 9.3 Securities Lending 30.5 — 30.5 (30.5 ) — — Total $ 82.7 $ — $ 82.7 $ (73.4 ) $ — $ 9.3
Net Investment Income	Net Investment Income Net investment income reported in our consolidated statements of income is as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Fixed Maturity Securities $ 557.1 $ 563.7 Derivatives 15.4 13.4 Mortgage Loans 27.3 25.4 Policy Loans 4.4 4.3 Other Long-term Investments Equity Securities 1 (0.2 ) 0.3 Private Equity Partnerships 1 5.6 5.1 Other 2.1 0.7 Short-term Investments 3.6 1.9 Gross Investment Income 615.3 614.8 Less Investment Expenses 9.6 8.8 Less Investment Income on Participation Fund Account Assets 3.4 3.6 Net Investment Income $ 602.3 $ 602.4 1 The net unrealized loss recognized in net investment income for the first three months of 2018 related to equity securities and private equity partnerships still held at March 31, 2018 was $2.0 million and $1.6 million , respectively.
Realized Investment Gains and Losses Reported in Consolidated Statements of Income	Realized Investment Gain and Loss Realized investment gains and losses are as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Fixed Maturity Securities Gross Gains on Sales $ 2.0 $ 2.4 Gross Losses on Sales (1.4 ) (1.1 ) Other-Than-Temporary Impairment Loss (1.0 ) — Mortgage Loans and Other Invested Assets Gross Gains on Sales — 0.7 Gross Losses on Sales — (0.2 ) Embedded Derivative in Modified Coinsurance Arrangement (1.7 ) 8.6 All Other Derivatives 0.7 (0.2 ) Foreign Currency Transactions (0.8 ) 0.8 Net Realized Investment Gain (Loss) $ (2.2 ) $ 11.0

Derivative Financial Instrume26

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2018
Notes to Financial Statements [Abstract]
Nature and Amount of Collateral Received From and Posted To Our Derivative Counterparties	The table below summarizes the nature and amount of collateral received from and posted to our derivative counterparties. March 31, 2018 December 31, 2017 (in millions of dollars) Carrying Value of Collateral Received from Counterparties Cash $ 16.2 $ 15.7 Carrying Value of Collateral Posted to Counterparties Fixed Maturity Securities $ 49.6 $ 46.4
Notional Amounts for Each Category of Derivative Activity	The table below summarizes, by notional amounts, the activity for each category of derivatives. The notional amounts represent the basis upon which our counterparty pay and receive amounts are calculated. Swaps Receive Variable/Pay Fixed Receive Fixed/Pay Fixed Receive Fixed/Pay Variable Credit Default Forwards Total (in millions of dollars) Balance at December 31, 2016 $ 105.5 $ 616.5 $ 250.0 $ 70.0 $ 10.0 $ 1,052.0 Additions — — — — — — Terminations 0.1 20.3 — — 10.0 30.4 Balance at March 31, 2017 $ 105.4 $ 596.2 $ 250.0 $ 70.0 $ — $ 1,021.6 Balance at December 31, 2017 $ 48.0 $ 536.5 $ 250.0 $ 70.0 $ — $ 904.5 Additions — 19.9 — — 26.8 46.7 Terminations — — — 70.0 — 70.0 Balance at March 31, 2018 $ 48.0 $ 556.4 $ 250.0 $ — $ 26.8 $ 881.2
Fair Value Hedges Carrying Amount of Hedged Assets and Liabilities and Cumulative Basis Adjustments	The following table summarizes the carrying amount of hedged assets and liabilities and the related cumulative basis adjustments related to our fair value hedges. Carrying Amount of Hedged Assets (Liabilities) Cumulative Amount of Fair Value Hedging Adjustment Included in the Carrying Amount of the Hedged Assets (Liabilities) March 31, 2018 December 31, 2017 March 31, 2018 December 31, 2017 (in millions of dollars) Fixed maturity securities: Receive variable interest rate, pay fixed interest rate $ 48.1 $ 48.5 $ 0.1 $ 0.6 Receive fixed foreign currency interest, pay fixed foreign currency interest 19.4 — (0.3 ) — Long-term Debt (242.5 ) (244.8 ) 6.9 4.5
Location and Fair Values of Derivative Financial Instruments	The following tables summarize the location and fair values of derivative financial instruments, as reported in our consolidated balance sheets. March 31, 2018 Derivative Assets Derivative Liabilities Balance Sheet Location Fair Value Balance Sheet Location Fair Value (in millions of dollars) Designated as Hedging Instruments Cash Flow Hedges Forwards Other L-T Investments $ — Other Liabilities $ 0.2 Foreign Exchange Contracts Other L-T Investments 21.2 Other Liabilities 17.4 Total Cash Flow Hedges 21.2 17.6 Fair Value Hedges Interest Rate Swaps Other L-T Investments — Other Liabilities 7.0 Foreign Exchange Contracts Other L-T Investments 0.3 Other Liabilities — Total Fair Value Hedges 0.3 7.0 Total Designated as Hedging Instruments $ 21.5 $ 24.6 Not Designated as Hedging Instruments Foreign Exchange Contracts Other Liabilities $ 26.8 Embedded Derivative in Modified Coinsurance Arrangement Other Liabilities 17.6 Total Not Designated as Hedging Instruments $ 44.4 December 31, 2017 Derivative Assets Derivative Liabilities Balance Sheet Location Fair Value Balance Sheet Location Fair Value (in millions of dollars) Designated as Hedging Instruments Cash Flow Hedges Foreign Exchange Contracts Other L-T Investments $ 19.5 Other Liabilities $ 19.4 Total Cash Flow Hedges 19.5 19.4 Fair Value Hedges Interest Rate Swaps Other L-T Investments — Other Liabilities 5.1 Total Fair Value Hedges — 5.1 Total Designated as Hedging Instruments $ 19.5 $ 24.5 Not Designated as Hedging Instruments Credit Default Swaps Other Liabilities $ 0.2 Foreign Exchange Contracts Other Liabilities 27.5 Embedded Derivative in Modified Coinsurance Arrangement Other Liabilities 15.9 Total Not Designated as Hedging Instruments $ 43.6
Location of Gains and Losses of Derivative Financial Instruments Designated as Hedging Instruments, Consolidated Statements of Income	The following table summarizes the location of gains and losses of derivative financial instruments designated as hedging instruments, as reported in our consolidated statements of income. Three Months Ended March 31 2018 2017 Net Investment Income Net Realized Investment Gain (Loss) Interest and Debt Expense Net Investment Income Net Realized Investment Gain (Loss) Interest and Debt Expense (in millions of dollars) Total Income and Expense Presented in the Consolidated Statements of Income of Which Hedged Items are Recorded $ 602.3 $ (2.2 ) $ 40.2 $ 602.4 $ 11.0 $ 39.8 Gain (Loss) on Cash Flow Hedging Relationships Interest Rate Swaps: Hedged items 77.8 0.1 11.4 81.9 0.2 11.4 Derivatives Designated as Hedging Instruments 15.9 (0.1 ) 0.6 14.6 — 0.5 Foreign Exchange Contracts Hedged items 5.0 — — 5.4 1.2 — Derivatives Designated as Hedging Instruments (0.3 ) — — (0.2 ) (1.2 ) — Gain (Loss) on Fair Value Hedging Relationships Interest Rate Swaps: Hedged items 0.7 1.9 3.6 1.4 (0.6 ) 3.6 Derivatives Designated as Hedging Instruments (0.4 ) (1.9 ) 0.1 (1.0 ) 0.6 (0.3 ) Foreign Exchange Contracts Hedged items 0.1 (0.3 ) — — — — Derivatives Designated as Hedging Instruments — 0.3 — — — —
Location of Gains and Losses of Derivative Financial Instruments Designated as Hedging Instruments, Statements of Comprehensive Income	The following table summarizes the location of gains and losses of derivative financial instruments designated as cash flow hedging instruments, as reported in our consolidated statements of comprehensive income. Three Months Ended March 31 2018 2017 (in millions of dollars) Gain (Loss) Recognized in Other Comprehensive Income (Loss) on Derivatives Interest Rate Swaps and Forwards $ (0.2 ) $ — Foreign Exchange Contracts 3.8 (4.5 ) Total $ 3.6 $ (4.5 )
Gains and Losses on Derivatives Not Designated as Hedging Instruments	The following table summarizes the location of gains and losses on our derivatives not designated as hedging instruments, as reported in our consolidated statements of income. Three Months Ended March 31 2018 2017 (in millions of dollars) Net Realized Investment Gain (Loss) Credit Default Swaps $ — $ (0.3 ) Foreign Exchange Contracts 0.7 0.2 Embedded Derivative in Modified Coinsurance Arrangement (1.7 ) 8.6 Total $ (1.0 ) $ 8.5

Accumulated Other Comprehensi27

Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)	Components of our accumulated other comprehensive income (loss), after tax, and related changes are as follows: Net Unrealized Gain on Securities Net Gain on Hedges Foreign Currency Translation Adjustment Unrecognized Pension and Postretirement Benefit Costs Total (in millions of dollars) Balance at December 31, 2017 $ 607.8 $ 282.3 $ (254.5 ) $ (508.1 ) $ 127.5 Adjustment to Adopt Accounting Standard Update - Note 2 (17.5 ) — — — (17.5 ) Balance at January 1, 2018 $ 590.3 $ 282.3 $ (254.5 ) $ (508.1 ) $ 110.0 Other Comprehensive Income (Loss) Before Reclassifications (234.8 ) 2.9 47.5 (1.2 ) (185.6 ) Amounts Reclassified from Accumulated Other Comprehensive Income or Loss 0.6 (11.7 ) — 4.3 (6.8 ) Net Other Comprehensive Income (Loss) (234.2 ) (8.8 ) 47.5 3.1 (192.4 ) Balance at March 31, 2018 $ 356.1 $ 273.5 $ (207.0 ) $ (505.0 ) $ (82.4 ) Balance at December 31, 2016 $ 440.6 $ 327.5 $ (354.0 ) $ (465.1 ) $ (51.0 ) Other Comprehensive Income (Loss) Before Reclassifications 44.7 (2.7 ) 17.1 (0.6 ) 58.5 Amounts Reclassified from Accumulated Other Comprehensive Income or Loss (2.3 ) (8.0 ) — 3.2 (7.1 ) Net Other Comprehensive Income (Loss) 42.4 (10.7 ) 17.1 2.6 51.4 Balance at March 31, 2017 $ 483.0 $ 316.8 $ (336.9 ) $ (462.5 ) $ 0.4
Schedule of Components of Unrealized Gain on Securities	The net unrealized gain on securities consists of the following components: March 31 January 1 2018 2018 Change (in millions of dollars) Fixed Maturity Securities $ 4,348.2 $ 5,665.2 $ (1,317.0 ) Deferred Acquisition Costs (41.2 ) (51.4 ) 10.2 Reserves for Future Policy and Contract Benefits (4,030.1 ) (5,094.7 ) 1,064.6 Reinsurance Recoverable 321.0 375.8 (54.8 ) Income Tax (241.8 ) (304.6 ) 62.8 Total $ 356.1 $ 590.3 $ (234.2 ) March 31 December 31 2017 2016 Change (in millions of dollars) Fixed Maturity Securities $ 4,925.6 $ 4,664.6 $ 261.0 Other Investments (17.8 ) (22.7 ) 4.9 Deferred Acquisition Costs (40.9 ) (38.9 ) (2.0 ) Reserves for Future Policy and Contract Benefits (4,467.3 ) (4,253.2 ) (214.1 ) Reinsurance Recoverable 335.6 321.3 14.3 Income Tax (252.2 ) (230.5 ) (21.7 ) Total $ 483.0 $ 440.6 $ 42.4
Schedule of Reclassifications Out of Accumulated Other Comprehensive Income	Amounts reclassified from accumulated other comprehensive income or loss were recognized in our consolidated statements of income as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Net Unrealized Gain on Securities Net Realized Investment Gain (Loss) Net Gain on Sales of Securities and Other Invested Assets $ 0.2 $ 3.4 Other-Than-Temporary Impairment Loss (1.0 ) — (0.8 ) 3.4 Income Tax Expense (Benefit) (0.2 ) 1.1 Total $ (0.6 ) $ 2.3 Net Gain on Hedges Net Investment Income Gain on Interest Rate Swaps and Forwards $ 15.6 $ 14.3 Loss on Foreign Exchange Contracts (0.3 ) (0.3 ) Net Realized Investment Gain (Loss) Loss on Interest Rate Swaps (0.1 ) — Loss on Foreign Exchange Contracts — (1.2 ) Interest and Debt Expense Loss on Interest Rate Swaps (0.5 ) (0.5 ) 14.7 12.3 Income Tax Expense 3.0 4.3 Total $ 11.7 $ 8.0 Unrecognized Pension and Postretirement Benefit Costs Other Expenses Amortization of Net Actuarial Loss $ (5.6 ) $ (5.1 ) Amortization of Prior Service Credit 0.1 0.2 (5.5 ) (4.9 ) Income Tax Benefit (1.2 ) (1.7 ) Total $ (4.3 ) $ (3.2 )

Liability for Unpaid Claims a28

Liability for Unpaid Claims and CAE Liability for Unpaid Claims (Tables)	3 Months Ended
	Mar. 31, 2018
Insurance [Abstract]
Schedule of Liability for Unpaid Claims and Claims Adjustment Expense	Changes in the liability for unpaid claims and claim adjustment expenses are as follows: 2018 2017 (in millions of dollars) Balance at January 1 $ 23,222.0 $ 23,249.5 Less Reinsurance Recoverable 2,182.0 2,163.6 Net Balance at January 1 21,040.0 21,085.9 Incurred Related to Current Year 1,543.1 1,489.8 Prior Years Interest 283.6 295.7 All Other Incurred (192.0 ) (184.8 ) Foreign Currency 72.2 29.8 Total Incurred 1,706.9 1,630.5 Paid Related to Current Year (322.0 ) (283.9 ) Prior Years (1,400.4 ) (1,369.3 ) Total Paid (1,722.4 ) (1,653.2 ) Net Balance at March 31 21,024.5 21,063.2 Plus Reinsurance Recoverable 2,184.1 2,139.1 Balance at March 31 $ 23,208.6 $ 23,202.3
Reconciliation of Policy and Contract Benefits and Reserves for Future Policy and Contract Benefits to Balance Sheet Amounts	A reconciliation of policy and contract benefits and reserves for future policy and contract benefits as reported in our consolidated balance sheets to the liability for unpaid claims and claim adjustment expenses is as follows: March 31 2018 2017 (in millions of dollars) Policy and Contract Benefits $ 1,623.1 $ 1,544.8 Reserves for Future Policy and Contract Benefits 44,668.5 44,502.2 Total 46,291.6 46,047.0 Less: Life Reserves for Future Policy and Contract Benefits 8,278.7 8,105.7 Accident and Health Active Life Reserves 10,774.2 10,271.7 Adjustment Related to Unrealized Investment Gains and Losses 4,030.1 4,467.3 Liability for Unpaid Claims and Claim Adjustment Expenses $ 23,208.6 $ 23,202.3 The adjustment related to unrealized investment gains and losses reflects the changes that would be necessary to policyholder liabilities if the unrealized investment gains and losses related to the corresponding available-for-sale securities had been realized. Changes in this adjustment are reported as a component of other comprehensive income or loss.

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Premium Income by Major Line of Business within Each Segment	Segment information is as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Premium Income Unum US Group Disability Group Long-term Disability $ 441.2 $ 434.4 Group Short-term Disability 176.3 157.9 Group Life and Accidental Death & Dismemberment Group Life 399.2 367.8 Accidental Death & Dismemberment 38.7 36.6 Supplemental and Voluntary Individual Disability 104.9 107.0 Voluntary Benefits 229.8 215.2 Dental and Vision 48.7 41.5 1,438.8 1,360.4 Unum UK Group Long-term Disability 90.8 80.8 Group Life 28.6 24.8 Supplemental 20.2 15.7 139.6 121.3 Colonial Life Accident, Sickness, and Disability 231.3 219.1 Life 81.0 74.2 Cancer and Critical Illness 86.0 81.0 398.3 374.3 Closed Block Individual Disability 109.4 121.3 Long-term Care 161.3 163.1 All Other 2.6 2.5 273.3 286.9 Total Premium Income $ 2,250.0 $ 2,142.9
Selected Operating Statement Data by Segment	Unum US Unum UK Colonial Life Closed Block Corporate Total (in millions of dollars) Three Months Ended March 31, 2018 Premium Income $ 1,438.8 $ 139.6 $ 398.3 $ 273.3 $ — $ 2,250.0 Net Investment Income 194.2 27.6 37.3 337.7 5.5 602.3 Other Income 29.0 — 0.3 19.0 1.2 49.5 Adjusted Operating Revenue $ 1,662.0 $ 167.2 $ 435.9 $ 630.0 $ 6.7 $ 2,901.8 Adjusted Operating Income (Loss) $ 243.9 $ 29.8 $ 81.0 $ 28.9 $ (40.3 ) $ 343.3 Three Months Ended March 31, 2017 Premium Income $ 1,360.4 $ 121.3 $ 374.3 $ 286.9 $ — $ 2,142.9 Net Investment Income 202.5 26.6 35.1 335.3 2.9 602.4 Other Income 28.7 — 0.3 20.8 0.4 50.2 Adjusted Operating Revenue $ 1,591.6 $ 147.9 $ 409.7 $ 643.0 $ 3.3 $ 2,795.5 Adjusted Operating Income (Loss) $ 239.1 $ 26.6 $ 82.4 $ 31.6 $ (39.8 ) $ 339.9
Assets by Segment	March 31 December 31 2018 2017 (in millions of dollars) Assets Unum US $ 18,036.3 $ 18,109.1 Unum UK 3,497.6 3,428.1 Colonial Life 4,292.9 4,184.1 Closed Block 34,603.8 35,051.2 Corporate 2,581.7 3,240.6 Total Assets $ 63,012.3 $ 64,013.1
Reconciliation of Total Revenue and Income before Income Tax by Segment to Adjusted Operating Revenue and Adjusted Operating Income	A reconciliation of total revenue to "adjusted operating revenue" and income before income tax to "adjusted operating income" is as follows: Three Months Ended March 31 2018 2017 (in millions of dollars) Total Revenue $ 2,899.6 $ 2,806.5 Excluding: Net Realized Investment Gain (Loss) (2.2 ) 11.0 Adjusted Operating Revenue $ 2,901.8 $ 2,795.5 Income Before Income Tax $ 341.1 $ 330.3 Excluding: Net Realized Investment Gain (Loss) (2.2 ) 11.0 Loss from Guaranty Fund Assessment — (20.6 ) Adjusted Operating Income $ 343.3 $ 339.9

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	3 Months Ended
	Mar. 31, 2018
Defined Benefit Plan [Abstract]
Schedule of Net Periodic Benefit Costs	The following table provides the components of the net periodic benefit cost (credit) for the defined benefit pension and OPEB plans. Three Months Ended March 31 Pension Benefits U.S. Plans U.K. Plan OPEB 2018 2017 2018 2017 2018 2017 (in millions of dollars) Service Cost $ 2.3 $ 2.0 $ — $ — $ — $ — Interest Cost 19.9 21.1 1.5 1.5 1.2 1.5 Expected Return on Plan Assets (26.1 ) (25.8 ) (2.4 ) (2.0 ) (0.1 ) (0.2 ) Amortization of: Net Actuarial Loss 5.4 4.9 0.2 0.2 — — Prior Service Credit — (0.1 ) — — (0.1 ) (0.1 ) Total $ 1.5 $ 2.1 $ (0.7 ) $ (0.3 ) $ 1.0 $ 1.2 The service cost component of net periodic pension and postretirement benefit cost is included as a component of compensation expense in our consolidated statements of income. All other components of net periodic pension and postretirement benefit cost are included in other expenses.

Stockholders' Equity and Earn31

Stockholders' Equity and Earnings Per Common Share (Tables)	3 Months Ended
	Mar. 31, 2018
Stockholders' Equity and Earnings Per Common Share [Abstract]
Schedule of Basic and Diluted Earnings Per Share	Earnings Per Common Share Net income per common share is determined as follows: Three Months Ended March 31 2018 2017 (in millions of dollars, except share data) Numerator Net Income $ 273.5 $ 229.9 Denominator (000s) Weighted Average Common Shares - Basic 221,894.0 229,429.6 Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards 683.0 949.2 Weighted Average Common Shares - Assuming Dilution 222,577.0 230,378.8 Net Income Per Common Share Basic $ 1.23 $ 1.00 Assuming Dilution $ 1.23 $ 1.00
Treasury Stock Transactions	Common Stock During the second quarter of 2017, our board of directors authorized the repurchase of up to $750.0 million of Unum Group's outstanding common stock through November 25, 2018. This authorization replaced the previous authorization of $750.0 million that was scheduled to expire on November 26, 2017. The remaining repurchase amount under the new program was $412.8 million at March 31, 2018 . Common stock repurchases, which are accounted for using the cost method and classified as treasury stock until otherwise retired, were as follows: Three Months Ended March 31 2018 2017 (in millions) Shares Repurchased 1.9 2.1 Cost of Shares Repurchased 1 $ 100.2 $ 100.0 1 Includes commissions of $0.2 million for the three months ended March 31, 2018 , and a de minimis amount for the three months ended March 31, 2017 .

Accounting Developments New Acc

Accounting Developments New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle
Increase (Decrease) in Accrued Investment Income Receivable	$ 95.2	$ 103.7
Proceeds from Sales and Maturities of Other Investments	82.3	46.5
Compensation Expense	221.7	209
Other Expenses	224.2	228.8
Other Long-term Investments	706.5			$ 646.8
Deferred Income Tax	128.9			199
Accumulated Other Comprehensive Income (Loss)	(82.4)			127.5
Retained Earnings	$ 9,777.8			9,542.2
Accounting Standards Update 2016-15
New Accounting Pronouncements or Change in Accounting Principle
Increase (Decrease) in Accrued Investment Income Receivable		4.4
Proceeds from Sales and Maturities of Other Investments		46.5
Accounting Standards Update 2016-15 | Scenario, Previously Reported
New Accounting Pronouncements or Change in Accounting Principle
Increase (Decrease) in Accrued Investment Income Receivable		2.2
Proceeds from Sales and Maturities of Other Investments		48.7
Accounting Standards Update 2016-15 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Increase (Decrease) in Accrued Investment Income Receivable		2.2
Proceeds from Sales and Maturities of Other Investments		(2.2)
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		209
Other Expenses		228.8
Accounting Standards Update 2017-07 | Scenario, Previously Reported
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		207
Other Expenses		230.8
Accounting Standards Update 2017-07 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Compensation Expense		2
Other Expenses		$ (2)
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle
Other Long-term Investments			$ 643	646.8
Deferred Income Tax			198.2	199
Accumulated Other Comprehensive Income (Loss)			110	127.5
Retained Earnings			9,556.7	$ 9,542.2
Accounting Standards Update 2016-01 | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Other Long-term Investments			(3.8)
Deferred Income Tax			(0.8)
Accumulated Other Comprehensive Income (Loss)			(17.5)
Retained Earnings			$ 14.5

Fair Values of Financial Inst33

Fair Values of Financial Instruments Private Equity Partnerships $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Investments Net Asset Value	$ 413.3
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	307.3
Private Credit
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Investments Net Asset Value	187.9	[1]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	118.1
Private Credit | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Investments Net Asset Value	$ 165.5	[1]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Description	Not redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 116.1
Private Credit | Quarterly after 2 year lock with 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Investments Net Asset Value	$ 22.4	[1]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Description	Initial 2 year lock on each new investment / Quarterly after 2 year lock with 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 2
Private Equity Funds | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Investments Net Asset Value	$ 101.3	[2]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Description	Not redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 106.2
Real Assets
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Investments Net Asset Value	124.1	[3]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	83
Real Assets | Not Redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Investments Net Asset Value	$ 93.9	[3]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Description	Not redeemable
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 83
Real Assets | Quarterly
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share
Investments Net Asset Value	$ 30.2	[3]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Description	Quarterly / 90 days notice
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unfunded Commitments	$ 0
[1]	Private Credit - The limited partnerships described in this category employ various investment strategies, generally providing direct lending or other forms of debt financing including first-lien, second-lien, mezzanine, and subordinated loans. The limited partnerships have credit exposure to corporates, physical assets, and/or financial assets within variety of industries (including manufacturing, healthcare, energy, business services, technology, materials, and retail) in North America and, to a lesser extent, outside of North America. Unless specifically disclosed in the table above, these limited partnerships do not allow for redemptions. As of March 31, 2018, the estimated remaining life of the investments that do not allow for redemptions is approximately 30 percent in the next 3 years, 42 percent during the period from 3 to 5 years, 23 percent during the period from 5 to 10 years, and 5 percent during the period from 10 to 15 years.
[2]	Private Equity - The limited partnerships described in this category employ various strategies generally investing in controlling or minority control equity positions directly in companies and/or assets across various industries (including manufacturing, healthcare, energy, business services, technology, materials, and retail), primarily in private markets within North America and, to a lesser extent, outside of North America. Unless specifically disclosed in the table above, these limited partnerships do not allow for redemptions. As of March 31, 2018, the estimated remaining life of the investments that do not allow for redemptions is approximately 39 percent in the next 3 years, 31 percent during the period from 3 to 5 years, 28 percent during the period from 5 to 10 years, and 2 percent during the period from 10 to 15 years.
[3]	Real Assets - The limited partnerships described in this category employ various strategies, which include investing in the equity and/or debt financing of physical assets, including infrastructure (energy, power, water/wastewater, communications), transportation (including airports, ports, toll roads, aircraft, railcars) and real estate in North America, Europe, South America, and Asia. Unless specifically disclosed in the table above, these limited partnerships do not allow for redemption. As of March 31, 2018, the estimated remaining life of the investments that do not allow for redemptions is approximately 3 percent in the next 3 years, 95 percent during the period from 5 to 10 years, and 2 percent during the period from 10 to 15 years.

Fair Value Measurement, Recurri

Fair Value Measurement, Recurring (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	$ 44,500.1
Derivative Asset	21.5	$ 19.5
Investments Net Asset Value	413.3
Other Long-term Investments	706.5	646.8
Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	44,500.1	45,457.8
Marketable Securities, Equity Securities	72.4	11.7
Other Long-term Investments	507.2	438.4
Assets, Fair Value Disclosure, Recurring	45,007.3	45,896.2
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	69	68.1
Liabilities, Fair Value Disclosure, Recurring	69	68.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	9,196.6	4,170.4
Marketable Securities, Equity Securities	24.9	0.2
Other Long-term Investments	24.9	0.2
Assets, Fair Value Disclosure, Recurring	9,221.5	4,170.6
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Liabilities, Fair Value Disclosure, Recurring	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	33,941.4	39,906.8
Marketable Securities, Equity Securities	46.4	10.4
Other Long-term Investments	67.9	29.9
Assets, Fair Value Disclosure, Recurring	34,009.3	39,936.7
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	51.4	52.2
Liabilities, Fair Value Disclosure, Recurring	51.4	52.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	1,362.1	1,380.6
Marketable Securities, Equity Securities	1.1	1.1
Other Long-term Investments	1.1	1.1
Assets, Fair Value Disclosure, Recurring	1,363.2	1,381.7
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	17.6	15.9
Liabilities, Fair Value Disclosure, Recurring	17.6	15.9
Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
Marketable Securities, Equity Securities	0	0
Other Long-term Investments	413.3	407.2
Assets, Fair Value Disclosure, Recurring	413.3	407.2
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Liabilities, Fair Value Disclosure, Recurring	0	0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	21.5	19.5
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	44.2	46.9
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	21.5	19.5
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	44.2	46.9
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Foreign Exchange Contracts | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Interest Rate Swaps | Fair Value, Measurements, Recurring
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	7.2	5.1
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	7.2	5.1
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Interest Rate Swaps | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Credit Default Swaps | Fair Value, Measurements, Recurring
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability		0.2
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability		0
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability		0.2
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability		0
Credit Default Swaps | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability		0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	17.6	15.9
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	17.6	15.9
Embedded Derivative in Modified Coinsurance Arrangement | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Equity Method Investments | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Investments Net Asset Value	413.3	407.2
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Investments Net Asset Value	0	0
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Investments Net Asset Value	0	0
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Investments Net Asset Value	0	0
Equity Method Investments | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure [Abstract]
Investments Net Asset Value	413.3	407.2
United States Government and Government Agencies and Authorities
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	1,546.3	1,482.5
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	1,546.3	1,482.5
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	495.3	460.1
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	1,051	1,022.4
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
United States Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
States, Municipalities, and Political Subdivisions
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	2,357.3	2,336.9
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	2,357.3	2,336.9
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	2,321.6	2,336.9
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	35.7	0
States, Municipalities, and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
Foreign Governments
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	873.7	863.9
Foreign Governments | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	873.7	863.9
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	841.4	863.9
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	32.3	0
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
Public Utilities
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	7,981.7	8,236.5
Public Utilities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	7,981.7	8,236.5
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	574.1	154.2
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	7,123.4	7,874.6
Public Utilities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	284.2	207.7
Public Utilities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
Mortgage/Asset-Backed Securities
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	1,842.8	1,973.6
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	1,842.8	1,973.6
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	1,842.3	1,973.6
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0.5	0
Mortgage/Asset-Backed Securities | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
All Other Corporate Bonds | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	29,857	30,522.4
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	8,127.2	3,556.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	20,742.6	25,816.2
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	987.2	1,150.1
All Other Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
Redeemable Preferred Stocks
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	41.3	42
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	41.3	42
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	0	0
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	19.1	19.2
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	22.2	22.8
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value Measured at Net Asset Value Per Share
Assets, Fair Value Disclosure [Abstract]
Fixed Maturity Securities	$ 0	$ 0

Transfers of Assets between Lev

Transfers of Assets between Level 1 and Level 2 (Detail) - First Quarter - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Transfers Between Level 1 and Level 2
Level 1 from Level 2	$ 5,843.5	$ 7,475.4
Level 2 from Level 1	1,015.3	1,312.8
United States Government and Government Agencies and Authorities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	40.1	54.5
Level 2 from Level 1	0	180
Public Utilities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	528.5	1,103.9
Level 2 from Level 1	62	57.5
All Other Corporate Bonds
Transfers Between Level 1 and Level 2
Level 1 from Level 2	5,274.9	6,317
Level 2 from Level 1	953.3	1,075.3
Equity Securities
Transfers Between Level 1 and Level 2
Level 1 from Level 2	24.2	0
Level 2 from Level 1	$ 14.9	$ 0

Changes in Assets and Liabiliti

Changes in Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Embedded Derivative in Modified Coinsurance Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Derivatives Measured with Unobservable Inputs, Beginning Balance	$ (15.9)	$ (46.7)
Derivative Liability Gain (Loss) Included in Earnings	(1.7)	8.6
Derivative Liability Gain (Loss) Included in OCI	0	0
Derivative Purchases	0	0
Derivative Sales	0	0
Derivative Transfers Into Level 3	0	0
Derivative Transfers Out of Level 3	0	0
Derivatives Measured with Unobservable Inputs, Ending Balance	(17.6)	(38.1)
States, Municipalities, and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	0	89.5
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	(0.8)	0
Investment Purchases	0	0
Investment Sales	(0.1)	0
Investment Level 3 Transfers Into	36.6	0.4
Investment Level 3 Transfers Out of	0	(89.5)
Assets Measured with Unobservable Inputs, Ending Balance	35.7	0.4
Foreign Governments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	0
Investment Gain (Loss) included in Earnings	0
Investment Gain (Loss) included in OCI	(0.4)
Investment Purchases	0
Investment Sales	0
Investment Level 3 Transfers Into	32.7
Investment Level 3 Transfers Out of	0
Assets Measured with Unobservable Inputs, Ending Balance	32.3
Public Utilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	207.7	265.3
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	(6.7)	1.8
Investment Purchases	0	8
Investment Sales	(1.1)	0
Investment Level 3 Transfers Into	200.9	112.6
Investment Level 3 Transfers Out of	(116.6)	(75.2)
Assets Measured with Unobservable Inputs, Ending Balance	284.2	312.5
Mortgage/Asset-Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	0
Investment Gain (Loss) included in Earnings	0
Investment Gain (Loss) included in OCI	0
Investment Purchases	0
Investment Sales	0
Investment Level 3 Transfers Into	0.5
Investment Level 3 Transfers Out of	0
Assets Measured with Unobservable Inputs, Ending Balance	0.5
All Other Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,150.1	1,459.7
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	(22.1)	13.1
Investment Purchases	0	0
Investment Sales	(11.7)	(36.4)
Investment Level 3 Transfers Into	466	143.9
Investment Level 3 Transfers Out of	(595.1)	(657.3)
Assets Measured with Unobservable Inputs, Ending Balance	987.2	923
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	22.8	23.2
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	(0.6)	(0.3)
Investment Purchases	0	0
Investment Sales	0	0
Investment Level 3 Transfers Into	0	0
Investment Level 3 Transfers Out of	0	0
Assets Measured with Unobservable Inputs, Ending Balance	22.2	22.9
Fixed Maturity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1,380.6	1,837.7
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	(30.6)	14.6
Investment Purchases	0	8
Investment Sales	(12.9)	(36.4)
Investment Level 3 Transfers Into	736.7	256.9
Investment Level 3 Transfers Out of	(711.7)	(822)
Assets Measured with Unobservable Inputs, Ending Balance	1,362.1	1,258.8
Equity Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation
Assets Measured with Unobservable Inputs, Beginning Balance	1.1	1.2
Investment Gain (Loss) included in Earnings	0	0
Investment Gain (Loss) included in OCI	0	0
Investment Purchases	0	0
Investment Sales	0	0
Investment Level 3 Transfers Into	0	0
Investment Level 3 Transfers Out of	0	0
Assets Measured with Unobservable Inputs, Ending Balance	$ 1.1	$ 1.2

Quantitative Information Regard

Quantitative Information Regarding Significant Unobservable Inputs (Details) - Fair Value, Inputs, Level 3 - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Embedded Derivative Liability
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Fair Value of Financial Instrument Using Internally Derived Unobservable Inputs		$ (17.6)	$ (15.9)
Embedded Derivative Liability | Projected Cash Flows
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Significant Assumptions to Calculating Fair Value of Level 3 Instrument	[1]	Actuarial Assumptions	Actuarial Assumptions
All Other Corporate Bonds - Private
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Fair Value of Financial Instrument Using Internally Derived Unobservable Inputs		$ 276.6	$ 244.4
All Other Corporate Bonds - Private | Market Convention
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Significant Assumptions to Calculating Fair Value of Level 3 Instrument	[2]	Priced at Par	Priced at Par
All Other Corporate Bonds - Private | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Comparability Adjustments	[3]	1.00%	0.20%
Lack of Marketability	[4]	0.25%	0.25%
Volatility of Credit	[5]	0.20%	0.12%
All Other Corporate Bonds - Private | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Comparability Adjustments	[3]	1.25%	0.20%
Lack of Marketability	[4]	0.25%	0.25%
Volatility of Credit	[5]	5.82%	6.25%
All Other Corporate Bonds - Private | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Comparability Adjustments	[3]	1.04%	0.20%
Lack of Marketability	[4]	0.25%	0.25%
Volatility of Credit	[5]	0.59%	0.50%
Equity Securities
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Fair Value of Financial Instrument Using Internally Derived Unobservable Inputs		$ 1.1	$ 1.1
Equity Securities | Market Convention
Fair Value Inputs, Assets and Liabilities, Quantitative Information
Significant Assumptions to Calculating Fair Value of Level 3 Instrument	[2]	Priced at Cost or Owner's Equity	Priced at Cost or Owner's Equity
[1]	Represents various actuarial assumptions required to derive the liability cash flows including incidence, termination, and lapse rates
[2]	Represents a decision to price based on par value, cost, or owner's equity when limited data is available
[3]	Represents basis point adjustments for changes in benchmark spreads associated with various industry sectors
[4]	Represents basis point adjustments to apply a discount due to the illiquidity of an investment
[5]	Represents basis point adjustments for credit-specific factors

Fair Values of Financial Inst38

Fair Values of Financial Instruments Fair Value, by Balance Sheet Groupings (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage Loans, Fair Value	$ 2,235.1	$ 2,306.2
Mortgage Loans, Carrying Value	2,200.8	2,213.2
Policy Loans, Fair Value	3,666	3,677.5
Policy Loans, Carrying Value	3,573.3	3,571.1
Other Long-term Investments
Miscellaneous Long-Term Investments, Fair Value	153.2	162.3
Miscellaneous Long-Term Investments, Carrying Value	153.2	162.3
Total Financial Instruments Assets Not Carried at Fair Value	6,054.3	6,146
Financial Instruments, Financial Assets Fair Value Disclosure, Carrying Value	5,927.3	5,946.6
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term Debt, Fair Value	2,961.6	3,048.7
Long-term Debt, Carrying Value	2,721.9	2,738.4
Collateralized Financings [Abstract]
Federal Home Loan Bank (FHLB) Funding Agreements, Fair Value	350	350
Federal Home Loan Bank (FHLB) Funding Agreements, Carrying Value	350	350
Other Liabilities, Unfunded Commitments	3.7	3.7
Total Financial Liabilities, Fair Value	3,315.3	3,402.4
Total Financial Liabilities, Carrying Value	3,075.6	3,092.1
Fair Value, Inputs, Level 1
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage Loans, Fair Value	0	0
Policy Loans, Fair Value	0	0
Other Long-term Investments
Miscellaneous Long-Term Investments, Fair Value	0	0
Total Financial Instruments Assets Not Carried at Fair Value	0	0
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term Debt, Fair Value	2,155.7	1,171.8
Collateralized Financings [Abstract]
Federal Home Loan Bank (FHLB) Funding Agreements, Fair Value	0	0
Other Liabilities, Unfunded Commitments	0	0
Total Financial Liabilities, Fair Value	2,155.7	1,171.8
Fair Value, Inputs, Level 2
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage Loans, Fair Value	2,235.1	2,306.2
Policy Loans, Fair Value	0	0
Other Long-term Investments
Miscellaneous Long-Term Investments, Fair Value	34.1	34.1
Total Financial Instruments Assets Not Carried at Fair Value	2,269.2	2,340.3
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term Debt, Fair Value	805.9	1,876.9
Collateralized Financings [Abstract]
Federal Home Loan Bank (FHLB) Funding Agreements, Fair Value	350	350
Other Liabilities, Unfunded Commitments	3.7	3.7
Total Financial Liabilities, Fair Value	1,159.6	2,230.6
Fair Value, Inputs, Level 3
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage Loans, Fair Value	0	0
Policy Loans, Fair Value	3,666	3,677.5
Other Long-term Investments
Miscellaneous Long-Term Investments, Fair Value	119.1	128.2
Total Financial Instruments Assets Not Carried at Fair Value	3,785.1	3,805.7
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term Debt, Fair Value	0	0
Collateralized Financings [Abstract]
Federal Home Loan Bank (FHLB) Funding Agreements, Fair Value	0	0
Other Liabilities, Unfunded Commitments	0	0
Total Financial Liabilities, Fair Value	$ 0	$ 0

Fair Value of Financial Instrum

Fair Value of Financial Instruments Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Gain (Loss) on Embedded Derivative	$ (1.7)	$ 8.6
Fair Value of Financial Instruments Additional Information [Abstract]
Ceded Policy Loans	$ 3,306.8		$ 3,307.5
Fair Value Inputs Other Than Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Percentage of Total Fair Value of Fixed Maturities Securities	79.30%
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Percentage of Total Fair Value of Fixed Maturities Securities	20.70%
Fair Value, Inputs, Level 2 | Pricing Service
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Percentage of Total Fair Value of Fixed Maturities Securities	66.00%
Fair Value, Inputs, Level 2 | Other Observable Market Data
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Percentage of Total Fair Value of Fixed Maturities Securities	2.40%
Fair Value Inputs Level 2 Or Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Percentage of Total Fair Value of Fixed Maturities Securities	10.90%

Amortized Cost and Fair Values

Amortized Cost and Fair Values of Securities by Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	$ 40,151.9	$ 39,780.5
Accumulated Gross Unrealized Gain on Securities	4,641.3
Accumulated Gross Unrealized Loss on Securities	293.1
Fixed Maturity Securities	44,500.1
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,411	1,311.1
Accumulated Gross Unrealized Gain on Securities	146.4	176.1
Accumulated Gross Unrealized Loss on Securities	11.1	4.7
Fixed Maturity Securities	1,546.3	1,482.5
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	2,016.6	1,942.8
Accumulated Gross Unrealized Gain on Securities	343.2	395.4
Accumulated Gross Unrealized Loss on Securities	2.5	1.3
Fixed Maturity Securities	2,357.3	2,336.9
Foreign Governments
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	692.6	673
Accumulated Gross Unrealized Gain on Securities	181.9	191.3
Accumulated Gross Unrealized Loss on Securities	0.8	0.4
Fixed Maturity Securities	873.7	863.9
Public Utilities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	6,919.6	6,952.7
Accumulated Gross Unrealized Gain on Securities	1,089.7	1,296.4
Accumulated Gross Unrealized Loss on Securities	27.6	12.6
Fixed Maturity Securities	7,981.7	8,236.5
Mortgage/Asset-Backed Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	1,776.4	1,873.2
Accumulated Gross Unrealized Gain on Securities	81.1	105.1
Accumulated Gross Unrealized Loss on Securities	14.7	4.7
Fixed Maturity Securities	1,842.8	1,973.6
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	27,296.7	26,988.7
Accumulated Gross Unrealized Gain on Securities	2,796.7	3,633.5
Accumulated Gross Unrealized Loss on Securities	236.4	99.8
Fixed Maturity Securities	29,857	30,522.4
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	39	39
Accumulated Gross Unrealized Gain on Securities	2.3	3
Accumulated Gross Unrealized Loss on Securities	0	0
Fixed Maturity Securities	41.3	42
Fixed Maturity Securities
Schedule of Available-for-sale Securities
Amortized Cost of Fixed Maturity Securities	40,151.9	39,780.5
Accumulated Gross Unrealized Gain on Securities	4,641.3	5,800.8
Accumulated Gross Unrealized Loss on Securities	293.1	123.5
Fixed Maturity Securities	$ 44,500.1	$ 45,457.8

Length of Time Fixed Maturity S

Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	$ 6,495.3	$ 2,037.6
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	187.1	40
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	1,093.2	1,144.5
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	106	83.5
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	178.4	157.9
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	8	3
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	57.5	58.8
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	3.1	1.8
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	134.3	45.7
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	1.4	0.5
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	24.2	22.3
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	1.1	0.7
Foreign Governments
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	38.8	13.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	0.8	0.4
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	0	0
Public Utilities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	400.7	213.2
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	17.8	7.9
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	152.2	133.5
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	9.8	4.7
Mortgage/Asset-Backed Securities
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	366.3	252.5
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	9.4	1.4
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	114.1	144.7
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	5.3	3.3
All Other Corporate Bonds
Schedule of Available-for-sale Securities
Fair Value of Securities in Continuous Unrealized Loss Possition for Less Than 12 Months	5,376.8	1,355.1
Available-for-sale Securities, Continuous Unrealized Loss Position of Less than 12 Months, Accumulated Loss	149.7	26.8
Fair Value of Securities in Continuous Unrealized Loss Possition for Greater Than 12 Months	745.2	785.2
Available-for-sale Securities, Continuous Unrealized Loss Position of 12 Months or Longer, Accumulated Loss	$ 86.7	$ 73

Distribution of the Maturity Da

Distribution of the Maturity Dates for Fixed Maturity Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Available-for-Sale Securities, Total Amortized Cost
1 year or less	$ 1,694.9	$ 1,625.1
Over 1 year through 5 years	5,665	5,579.9
Over 5 years through 10 years	12,273.8	12,091.1
Over 10 years	18,741.8	18,611.2
Available-for-sale Securities, Debt Maturities, Gross Subtotal	38,375.5	37,907.3
Mortgage/Asset-Backed Securities	1,776.4	1,873.2
Amortized Cost of Fixed Maturity Securities	40,151.9	39,780.5
Available-for-Sale Securities, Unrealized Gain Position, Gross Gain
1 year or less	23.4	30.9
Over 1 year through 5 years	372	453.6
Over 5 years through 10 years	903.8	1,169.8
Over 10 years	3,261	4,041.4
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Gain, Gross Subtotal	4,560.2	5,695.7
Mortgage/Asset-Backed Securities	81.1	105.1
Total Fixed Maturity Securities	4,641.3	5,800.8
Fair Value Maturity Distribution
Fixed Maturity Securities	44,500.1
Available-for-Sale Securities, Unrealized Loss Position, Gross Loss
1 year or less	4.1	3.1
Over 1 year through 5 years	22.3	18.5
Over 5 years through 10 years	138.1	53.2
Over 10 years	113.9	44
Available-For-Sale Securities, Debt Maturities Unrealized Gain Position, Gross Loss, Gross Subtotal	278.4	118.8
Mortgage/Asset-Backed Securities	14.7	4.7
Total Fixed Maturity Securities	293.1	123.5
Fair Value of Fixed Maturity Securities in Unrealized Gain Position
Fair Value Maturity Distribution
1 year or less	1,645	1,638.8
Over 1 year through 5 years	5,680.4	5,810.9
Over 5 years through 10 years	9,124.4	11,916.5
Over 10 years	19,099.4	21,333.1
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	35,549.2	40,699.3
Mortgage/Asset-Backed Securities	1,362.4	1,576.4
Fixed Maturity Securities	36,911.6	42,275.7
Fair Value of Fixed Maturity Securities in Unrealized Loss Position
Fair Value Maturity Distribution
1 year or less	69.2	14.1
Over 1 year through 5 years	334.3	204.1
Over 5 years through 10 years	3,915.1	1,291.2
Over 10 years	2,789.5	1,275.5
Available-for-sale Securities, Debt Maturities, Fair Value, Gross Subtotal	7,108.1	2,784.9
Mortgage/Asset-Backed Securities	480.4	397.2
Fixed Maturity Securities	$ 7,588.5	$ 3,182.1

Investments Distribution by Ext

Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) $ in Millions	Mar. 31, 2018USD ($)
Schedule of Available-for-sale Securities
Fixed Maturity Securities	$ 44,500.1
Accumulated Gross Unrealized Gain on Securities	4,641.3
Accumulated Gross Unrealized Loss on Securities	$ 293.1
Percent of Fixed Maturity Securities in Unrealized Loss Position	100.00%
External Credit Rating, Investment Grade
Schedule of Available-for-sale Securities
Fixed Maturity Securities	$ 41,233.8
Accumulated Gross Unrealized Gain on Securities	4,527.8
Accumulated Gross Unrealized Loss on Securities	$ 198.7
Percent of Fixed Maturity Securities in Unrealized Loss Position	67.80%
External Credit Rating, Non Investment Grade
Schedule of Available-for-sale Securities
Fixed Maturity Securities	$ 3,266.3
Accumulated Gross Unrealized Gain on Securities	113.5
Accumulated Gross Unrealized Loss on Securities	$ 94.4
Percent of Fixed Maturity Securities in Unrealized Loss Position	32.20%

Investments Low Income Housing

Investments Low Income Housing Tax Credits (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investments, All Other Investments [Abstract]
Low Income Housing Tax Credits	$ 10.4	$ 10.4
Amortization of Low Income Housing Investments	(7)	(5.8)
Tax Benefits from Low Income Housing Investments	$ 3.4	$ 4.6

Mortgage Loans by Property Type

Mortgage Loans by Property Type and Geographic Region (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Mortgage Loans on Real Estate
Mortgage Loans	$ 2,200.8	$ 2,213.2
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate
Mortgage Loans	$ 55.2	$ 56.1
Percent of Total	2.50%	2.50%
Middle Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 154.5	$ 155.5
Percent of Total	7.00%	7.00%
East North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 291.3	$ 282
Percent of Total	13.20%	12.80%
West North Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 208.4	$ 210.1
Percent of Total	9.50%	9.50%
South Atlantic
Mortgage Loans on Real Estate
Mortgage Loans	$ 481.7	$ 494.4
Percent of Total	21.90%	22.30%
East South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 88.1	$ 88.8
Percent of Total	4.00%	4.00%
West South Central
Mortgage Loans on Real Estate
Mortgage Loans	$ 245.4	$ 247.4
Percent of Total	11.20%	11.20%
Mountain
Mortgage Loans on Real Estate
Mortgage Loans	$ 254.9	$ 251.2
Percent of Total	11.60%	11.40%
Pacific
Mortgage Loans on Real Estate
Mortgage Loans	$ 421.3	$ 427.7
Percent of Total	19.10%	19.30%
Apartment
Mortgage Loans on Real Estate
Mortgage Loans	$ 380	$ 360
Percent of Total	17.30%	16.30%
Industrial
Mortgage Loans on Real Estate
Mortgage Loans	$ 584.9	$ 601.2
Percent of Total	26.60%	27.20%
Office
Mortgage Loans on Real Estate
Mortgage Loans	$ 674.5	$ 692.3
Percent of Total	30.60%	31.30%
Retail
Mortgage Loans on Real Estate
Mortgage Loans	$ 529.5	$ 527.6
Percent of Total	24.10%	23.80%
Other Property
Mortgage Loans on Real Estate
Mortgage Loans	$ 31.9	$ 32.1
Percent of Total	1.40%	1.40%

Mortgage Loans Sorted by Applic

Mortgage Loans Sorted by Applicable Internal Credit Ratings and Loan-to-Value Ratios (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment
Mortgage Loans	$ 2,200.8	$ 2,213.2
Aa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	0.3	0.4
A Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	481.9	445.7
Baa Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	1,704.4	1,753
Ba Credit Rating
Financing Receivable, Recorded Investment
Mortgage Loans	14.2	14.1
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	1,082.6	1,101.7
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	1,053.8	1,041.6
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	40.2	49.3
Loan To Value Ratio Above 85 Percent
Financing Receivable, Recorded Investment
Mortgage Loans	$ 24.2	$ 20.6

Investments Remaining Contractu

Investments Remaining Contractual Maturity of Securities Lending Agreements (Details) - Overnight and Continuous - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Transfer of Certain Financial Assets Accounted for as Secured Borrowings
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	$ 16	$ 30.5
Securities Loaned and Securities Sold under Agreement to Repurchase, Gross Including Not Subject to Master Netting Arrangement	16	30.5
Secured Borrowings, Gross, Difference, Amount	0	0
United States Government and Government Agencies and Authorities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	1	0.2
Public Utilities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	0	0.5
All Other Corporate Bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings
Securities Loaned, Including Not Subject to Master Netting Arrangement and Assets other than Securities Transferred	$ 15	$ 29.8

Investments Federal Home Loan B

Investments Federal Home Loan Bank Carrying Value of Common Stock, Collateral Posted and Advances Received (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Federal Home Loan Bank Information
Federal Home Loan Bank Stock	$ 34.1	$ 34.1
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	350	350
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	590.1	544.8
Fixed Maturity Securities
Schedule of Federal Home Loan Bank Information
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	268.5	213.3
Mortgage Loans as Collateral
Schedule of Federal Home Loan Bank Information
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	$ 321.6	$ 331.5

Schedule of Financial Instrumen

Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Offsetting Derivative Assets
Gross Derivative Assets	$ 21.5	$ 19.5
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0	0
Net Derivative Assets Reported in Balance Sheet	21.5	19.5
Securities Received as Collateral on Derivative Assets	(5.4)	(4.2)
Cash Collateral from Counterparties	(16.1)	(15.3)
Derivative Assets Net of Collateral	0	0
Offsetting Securities Borrowed
Gross Securities Borrowed	104.2	159.2
Liabilities Offsetting Securities Borrowed in Balance Sheet	0	0
Net Securities Borrowed Reported in Balance Sheet	104.2	159.2
Securities Received as Collateral on Securities Borrowed	(88.2)	(128.7)
Cash Received as Collateral on Securities Borrowed	(16)	(30.5)
Securities Borrowed Net of Collateral	0	0
Offsetting of Derivative Assets and Securities Borrowed
Gross Financial Assets	125.7	178.7
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0	0
Net Financial Assets Reported in Balance Sheet	125.7	178.7
Securities Received as Collateral on Financial Assets	(93.6)	(132.9)
Cash Received as Collateral on Financial Assets	(32.1)	(45.8)
Financial Assets Net of Collateral	0	0
Offsetting Securities Loaned
Gross Securities Loaned	16	30.5
Assets Offsetting Securities Loaned in Balance Sheet	0	0
Net Securities Loaned Reported in Balance Sheet	16	30.5
Securities Given as Collateral on Securities Loaned	(16)	(30.5)
Cash Given as Collateral on Securities Loaned	0	0
Securities Loaned Net of Collateral	0	0
Derivative Liabilities and Securities Lending Liabilities
Gross Financial Liability	67.4	82.7
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0	0
Net Financial Liabilities Reported in Balance Sheet	67.4	82.7
Securities Given as Collateral on Financial Liabilities	(55)	(73.4)
Cash Given as Collateral on Financial Liabilities	0	0
Financial Liabilities Net of Collateral	12.4	9.3
Over the Counter
Offsetting Derivative Liabilities
Gross Derivative Liability	51.4	52.2
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0	0
Net Derivative Liabilities Reported in Balance Sheet	51.4	52.2
Cash Collateral to Counterparties	0	0
Securities Given as Collateral on Derivative Liabilities	(39)	(42.9)
Derivative Liabilities Net of Collateral	$ 12.4	$ 9.3

Net Investment Income (Detail)

Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 615.3		$ 614.8
Less Investment Expenses	9.6		8.8
Less Investment Income on PFA Assets	3.4		3.6
Net Investment Income	602.3		602.4
Fixed Maturity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	557.1		563.7
Derivatives
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	15.4		13.4
Mortgage Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	27.3		25.4
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	4.4		4.3
Equity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	(0.2)	[1]	0.3
Unrealized Loss on Securities	2
Partnership Interest
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	5.6	[1]	5.1
Unrealized Loss on Securities	1.6
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	2.1		0.7
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 3.6		$ 1.9
[1]	1 The net unrealized loss recognized in net investment income for the first three months of 2018 related to equity securities and private equity partnerships still held at March 31, 2018 was $2.0 million and $1.6 million, respectively.

Realized Investment Gains and L

Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fixed Maturity Securities
Gross Gains on Sales	$ 2	$ 2.4
Gross Losses on Sales	(1.4)	(1.1)
Other-Than-Temporary Impairment Loss	(1)	0
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	0	0.7
Gross Losses on Sales	0	(0.2)
Gain (Loss) on Embedded Derivative	(1.7)	8.6
Gain (Loss) on All Other Derivatives	0.7	(0.2)
Foreign Currency Transactions	(0.8)	0.8
Net Realized Investment Gain (Loss)	$ (2.2)	$ 11

Investments - Additional Inform

Investments - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2017USD ($)
Schedule of Investments
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	$ 0		$ 0
Commitment to Fund Private Placement Fixed Maturity Securities	213,500,000
Carrying Amount of Variable Interest Entity Investments	532,400,000		$ 535,400,000
Unfunded Commitments to Investment Partnerships	3,700,000
Fair Value of Fixed Maturity Securities	$ 44,500,100,000
Mortgage Loan Policy is not to Exceed a Loan-to-value Ratio, Percent	75.00%
Loan to Value Ratio Update Frequency	3 years
Mortgage Loan Inspection Frequency	2 years
Mortage Loan Policy Debt Service Coverage Ratio Lower Range	125.00%
Mortgage Loans Issuance, Term Years	25
Number of Financing Receivables on Nonaccrual Status	1		0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	$ 3,200,000
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans	0
Impaired Financing Receivable, Average Recorded Investment	$ 1,100,000	$ 0
Impaired Financing Receivable, with No Related Allowance, Interest Income, Accrual Method	0	0
Committments to Fund Commercial Mortgage Loans	$ 78,900,000
Repurchase agreements - Typical Days Outstanding	30 days
Minimum Percent of Fair Value of Securities Loaned or Securities Purchased Under Repurchase Agreements to be Maintained as Collateral	102.00%
Net Securities Borrowed Reported in Balance Sheet	$ 104,200,000		$ 159,200,000
Outstanding Repurchase Agreements	0		0
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	16,000,000		30,500,000
Securities Received as Collateral	94,300,000		135,600,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Financing Receivable, Allowance for Credit Losses	0		0
Provision for Loan and Lease Losses	$ 0	$ 0
Commercial Real Estate Portfolio Segment
Schedule of Investments
Number of Loan Modifications due to Loan Foreclosure	0	0
Special Purpose Entity | Bonds
Schedule of Investments
Fair Value of Fixed Maturity Securities	$ 154,400,000		154,100,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	119,100,000		128,200,000
Equity Method Investments
Schedule of Investments
Carrying Amount of Variable Interest Entity Investments	$ 413,300,000		$ 407,200,000
External Credit Rating, Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	413
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	141
Fair Value of Fixed Maturity Securities	$ 41,233,800,000
External Credit Rating, Non Investment Grade
Schedule of Investments
Number of Fixed Maturity Securities in an Unrealized Loss Position	88
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year	20
Fair Value of Fixed Maturity Securities	$ 3,266,300,000

Derivative Financial Instrume53

Derivative Financial Instruments Nature and Amounts of Collateral Received From and Posted To Our Derivative Counterparties (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative
Derivative, Collateral, Obligation to Return Cash	$ 16.1	$ 15.3
Available-for-sale Securities Pledged as Collateral	49.6	46.4
Total Amount
Derivative
Derivative, Collateral, Obligation to Return Cash	$ 16.2	$ 15.7

Notional Amounts for Each Categ

Notional Amounts for Each Category of Derivative Activity (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative
Balance at beginning of period	$ 904.5	$ 1,052
Additions	46.7	0
Terminations	70	30.4
Balance at ending of period	881.2	1,021.6
Credit Risk Contract | Credit Default Swaps
Derivative
Balance at beginning of period	70	70
Additions	0	0
Terminations	70	0
Balance at ending of period	0	70
Forwards
Derivative
Balance at beginning of period	0	10
Additions	26.8	0
Terminations	0	10
Balance at ending of period	26.8	0
Receive Variable/Pay Fixed | Interest Rate Swaps
Derivative
Balance at beginning of period	48	105.5
Additions	0	0
Terminations	0	0.1
Balance at ending of period	48	105.4
Receive Fixed/Pay Fixed | Interest Rate Swaps
Derivative
Balance at beginning of period	536.5	616.5
Additions	19.9	0
Terminations	0	20.3
Balance at ending of period	556.4	596.2
Receive Fixed/Pay Variable | Interest Rate Swaps
Derivative
Balance at beginning of period	250	250
Additions	0	0
Terminations	0	0
Balance at ending of period	$ 250	$ 250

Derivative Financial Instrume55

Derivative Financial Instruments Fair Value Hedges Carrying Amount of Hedged Assets and Liabilities and Cumulative Basis Adjustments (Details) - Interest Rate Swaps - Designated as Hedging Instrument - Fair Value Hedging - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Receive Variable/Pay Fixed | Fixed Maturity Securities
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge	$ 48.1	$ 48.5
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	0.1	0.6
Receive Fixed/Pay Fixed | Fixed Maturity Securities
Derivative Instruments and Hedging Activities Disclosures
Hedged Asset, Fair Value Hedge	19.4	0
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease)	(0.3)	0
Receive Fixed/Pay Variable | Long-term Debt
Derivative Instruments and Hedging Activities Disclosures
Hedged Liability, Fair Value Hedge	(242.5)	(244.8)
Hedged Liability, Fair Value Hedge, Cumulative Increase (Decrease)	$ 6.9	$ 4.5

Location and Fair Values of Der

Location and Fair Values of Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative
Asset Derivatives Fair Value	$ 21.5	$ 19.5
Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	21.5	19.5
Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	24.6	24.5
Not Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	44.4	43.6
Not Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	26.8	27.5
Not Designated as Hedging Instrument | Credit Risk Contract | Other Liabilities
Derivative
Liability Derivatives Fair Value		0.2
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement | Other Liabilities
Derivative
Liability Derivatives Fair Value	17.6	15.9
Cash Flow Hedging | Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	21.2	19.5
Cash Flow Hedging | Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	17.6	19.4
Cash Flow Hedging | Designated as Hedging Instrument | Forwards | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0
Cash Flow Hedging | Designated as Hedging Instrument | Forwards | Other Liabilities
Derivative
Liability Derivatives Fair Value	0.2
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	21.2	19.5
Cash Flow Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	17.4	19.4
Fair Value Hedging | Designated as Hedging Instrument | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.3	0
Fair Value Hedging | Designated as Hedging Instrument | Other Liabilities
Derivative
Liability Derivatives Fair Value	7	5.1
Fair Value Hedging | Designated as Hedging Instrument | Interest Rate Swaps | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0	0
Fair Value Hedging | Designated as Hedging Instrument | Interest Rate Swaps | Other Liabilities
Derivative
Liability Derivatives Fair Value	7	$ 5.1
Fair Value Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Long-term Investments
Derivative
Asset Derivatives Fair Value	0.3
Fair Value Hedging | Designated as Hedging Instrument | Foreign Exchange Contracts | Other Liabilities
Derivative
Liability Derivatives Fair Value	$ 0

Derivative Financial Instrume57

Derivative Financial Instruments Location of Gains and Losses of Derivative Financial Instruments Designated as Hedging Instruments, Consolidated Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss)
Net Investment Income	$ 602.3	$ 602.4
Net Realized Investment Gain (Loss)	(2.2)	11
Interest and Debt Expense	40.2	39.8
Derivative, Gain (Loss) on Derivative, Net	(0.7)	0.2
Interest Rate Swaps | Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	77.8	81.9
Derivative, Gain (Loss) on Derivative, Net	15.9	14.6
Interest Rate Swaps | Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.7	1.4
Derivative, Gain (Loss) on Derivative, Net	(0.4)	(1)
Interest Rate Swaps | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	0.1	0.2
Derivative, Gain (Loss) on Derivative, Net	(0.1)	0
Interest Rate Swaps | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	1.9	(0.6)
Derivative, Gain (Loss) on Derivative, Net	(1.9)	0.6
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	11.4	11.4
Derivative, Gain (Loss) on Derivative, Net	0.6	0.5
Interest Rate Swaps | Interest Expense | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	3.6	3.6
Derivative, Gain (Loss) on Derivative, Net	0.1	(0.3)
Foreign Exchange Contracts | Investment Income | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	5	5.4
Derivative, Gain (Loss) on Derivative, Net	(0.3)	(0.2)
Foreign Exchange Contracts | Investment Income | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Investment Income	0.1	0
Derivative, Gain (Loss) on Derivative, Net	0	0
Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	0	1.2
Derivative, Gain (Loss) on Derivative, Net	0	(1.2)
Foreign Exchange Contracts | Net Realized Investment Gain (Loss) | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Net Realized Investment Gain (Loss)	(0.3)	0
Derivative, Gain (Loss) on Derivative, Net	0.3	0
Foreign Exchange Contracts | Interest Expense | Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	0	0
Derivative, Gain (Loss) on Derivative, Net	0	0
Foreign Exchange Contracts | Interest Expense | Designated as Hedging Instrument | Fair Value Hedging
Derivative Instruments, Gain (Loss)
Interest and Debt Expense	0	0
Derivative, Gain (Loss) on Derivative, Net	$ 0	$ 0

Location of Gains and Losses on

Location of Gains and Losses on Derivative Financial Instruments Designated as Cash Flow Hedging Instruments (Detail) - Cash Flow Hedging - Designated as Hedging Instrument - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 3.6	$ (4.5)
Interest Rate Swaps and Forwards
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(0.2)	0
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 3.8	$ (4.5)

Gains and Losses on Derivatives

Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	$ 0.7	$ (0.2)
Net Realized Investment Gain (Loss) | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	(1)	8.5
Net Realized Investment Gain (Loss) | Not Designated as Hedging Instrument | Credit Default Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	0	(0.3)
Net Realized Investment Gain (Loss) | Not Designated as Hedging Instrument | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	0.7	0.2
Net Realized Investment Gain (Loss) | Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	$ (1.7)	$ 8.6

Derivative Financial Instrume60

Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Derivative
Aggregate Fair Value of all Derivative Instruments with Credit Risk-related Contingent Features in a Liability Position	$ 51.4		$ 52.2
Notional Amount of Derivatives	881.2	$ 1,021.6	904.5	$ 1,052
Approximate Amount of Net Deferred Gains on Derivative Instruments Expected to be Amortized During the Next Twelve Months	65.1
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	0	0
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as Fair Value Hedge	0	0
Cash Flow Hedging | Interest Rate Swaps and Forwards
Derivative
Notional Amount of Derivatives	343.9		343.9
Credit Default Swaps | Credit Risk Contract
Derivative
Notional Amount of Derivatives	0	70	70	70
Credit Default Swaps | Not Designated as Hedging Instrument | Credit Risk Contract
Derivative
Notional Amount of Derivatives			70
Credit Exposure
Derivative
Fair Value, Concentration of Risk, Derivative Instruments, Assets	0.1
Receive Variable/Pay Fixed | Interest Rate Swaps
Derivative
Notional Amount of Derivatives	48	105.4	48	105.5
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	0.5	1.1
Receive Variable/Pay Fixed | Fair Value Hedging | Interest Rate Swaps
Derivative
Notional Amount of Derivatives	48		48
Receive Fixed/Pay Variable | Interest Rate Swaps
Derivative
Notional Amount of Derivatives	250	250	250	250
Receive Fixed/Pay Variable | Fair Value Hedging | Interest Rate Swaps
Derivative
Notional Amount of Derivatives	250		250
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	2.4	0.5
Receive Fixed/Pay Fixed | Interest Rate Swaps
Derivative
Notional Amount of Derivatives	556.4	$ 596.2	536.5	$ 616.5
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	0.3
Receive Fixed/Pay Fixed | Fair Value Hedging | Interest Rate Swaps
Derivative
Notional Amount of Derivatives	19.9
Receive Fixed/Pay Fixed | Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative
Notional Amount of Derivatives	$ 192.6		$ 192.6

Schedule of Accumulated Other C

Schedule of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income, Net of Tax
Net Other Comprehensive Income (Loss)	$ (192.4)	$ 51.4
Balance at End of Period	9,499.5	9,107.4
Net Unrealized Gain on Securities
Accumulated Other Comprehensive Income, Net of Tax
Balance at Beginning of Period	607.8	440.6
Balance at Beginning of Period, as Adjusted	590.3
Other Comprehensive Income (Loss) Before Reclassifications, Net of Tax	(234.8)	44.7
Reclassification from Accumulated Other Comprehensive Income or Loss	0.6	(2.3)
Net Other Comprehensive Income (Loss)	(234.2)	42.4
Balance at End of Period	356.1	483
Accumulated Net Gain (Loss) from Designated or Qualifying Hedges
Accumulated Other Comprehensive Income, Net of Tax
Balance at Beginning of Period	282.3	327.5
Balance at Beginning of Period, as Adjusted	282.3
Other Comprehensive Income (Loss) Before Reclassifications, Net of Tax	2.9	(2.7)
Reclassification from Accumulated Other Comprehensive Income or Loss	(11.7)	(8)
Net Other Comprehensive Income (Loss)	(8.8)	(10.7)
Balance at End of Period	273.5	316.8
Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income, Net of Tax
Balance at Beginning of Period	(254.5)	(354)
Balance at Beginning of Period, as Adjusted	(254.5)
Other Comprehensive Income (Loss) Before Reclassifications, Net of Tax	47.5	17.1
Reclassification from Accumulated Other Comprehensive Income or Loss	0	0
Net Other Comprehensive Income (Loss)	47.5	17.1
Balance at End of Period	(207)	(336.9)
Unrecognized Pension and Postretirement Benefit Costs
Accumulated Other Comprehensive Income, Net of Tax
Balance at Beginning of Period	(508.1)	(465.1)
Balance at Beginning of Period, as Adjusted	(508.1)
Other Comprehensive Income (Loss) Before Reclassifications, Net of Tax	(1.2)	(0.6)
Reclassification from Accumulated Other Comprehensive Income or Loss	4.3	3.2
Net Other Comprehensive Income (Loss)	3.1	2.6
Balance at End of Period	(505)	(462.5)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income, Net of Tax
Balance at Beginning of Period	127.5	(51)
Cumulative Effect of New Accounting Principle in Period of Adoption	(17.5)	0
Balance at Beginning of Period, as Adjusted	110	(51)
Other Comprehensive Income (Loss) Before Reclassifications, Net of Tax	(185.6)	58.5
Reclassification from Accumulated Other Comprehensive Income or Loss	(6.8)	(7.1)
Net Other Comprehensive Income (Loss)	(192.4)	51.4
Balance at End of Period	(82.4)	$ 0.4
Restatement Adjustment | Accounting Standards Update 2016-01 | Net Unrealized Gain on Securities
Accumulated Other Comprehensive Income, Net of Tax
Cumulative Effect of New Accounting Principle in Period of Adoption	(17.5)
Restatement Adjustment | Accounting Standards Update 2016-01 | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges
Accumulated Other Comprehensive Income, Net of Tax
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Restatement Adjustment | Accounting Standards Update 2016-01 | Foreign Currency Translation Adjustment
Accumulated Other Comprehensive Income, Net of Tax
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Restatement Adjustment | Accounting Standards Update 2016-01 | Unrecognized Pension and Postretirement Benefit Costs
Accumulated Other Comprehensive Income, Net of Tax
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Restatement Adjustment | Accounting Standards Update 2016-01 | Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income, Net of Tax
Cumulative Effect of New Accounting Principle in Period of Adoption	$ (17.5)

Schedule of Components of Unrea

Schedule of Components of Unrealized Gain (Loss) on Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Change in Components of Net Unrealized Gain on Securities
Change in Net Unrealized Gain on Securities	$ (1,042.1)	$ 176.9
Fixed Maturity Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	5,665.2	4,664.6
Change in Net Unrealized Gain on Securities	(1,317)	261
End of Period Balance	4,348.2	4,925.6
Other Investments
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance		(22.7)
Change in Net Unrealized Gain on Securities		4.9
End of Period Balance		(17.8)
Deferred Acquisition Costs
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(51.4)	(38.9)
Change in Net Unrealized Gain on Securities	10.2	(2)
End of Period Balance	(41.2)	(40.9)
Reserve for Future Policy and Contract Benefits
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(5,094.7)	(4,253.2)
Change in Net Unrealized Gain on Securities	1,064.6	(214.1)
End of Period Balance	(4,030.1)	(4,467.3)
Reinsurance Recoverable
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	375.8	321.3
Change in Net Unrealized Gain on Securities	(54.8)	14.3
End of Period Balance	321	335.6
Income Tax
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	(304.6)	(230.5)
Change in Net Unrealized Gain on Securities	62.8	(21.7)
End of Period Balance	(241.8)	(252.2)
Net Unrealized Gain on Securities
Change in Components of Net Unrealized Gain on Securities
Beginning of Period Balance	590.3	440.6
Change in Net Unrealized Gain on Securities	(234.2)	42.4
End of Period Balance	$ 356.1	$ 483

Schedule of Reclassifications O

Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	$ (1.2)	$ 11
Other-Than-Temporary Impairment Loss	(1)	0
Net Investment Income	602.3	602.4
Interest and Debt Expense	(40.2)	(39.8)
Other Expenses	(224.2)	(228.8)
Income Before Income Tax	341.1	330.3
Total Income Tax	67.6	100.4
Net Income	273.5	229.9
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Unrealized Gain on Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	0.2	3.4
Other-Than-Temporary Impairment Loss	(1)	0
Income Before Income Tax	(0.8)	3.4
Total Income Tax	(0.2)	1.1
Net Income	(0.6)	2.3
Reclassification out of Accumulated Other Comprehensive Income or Loss | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	14.7	12.3
Total Income Tax	3	4.3
Net Income	11.7	8
Reclassification out of Accumulated Other Comprehensive Income or Loss | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Interest Rate Swaps
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	(0.1)	0
Net Investment Income	15.6	14.3
Interest and Debt Expense	(0.5)	(0.5)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Foreign Exchange Contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Net Realized Investment Gain (Loss), Excluding Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	0	(1.2)
Net Investment Income	(0.3)	(0.3)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Net Actuarial Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	(5.6)	(5.1)
Reclassification out of Accumulated Other Comprehensive Income or Loss | Prior Service Credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Other Expenses	0.1	0.2
Reclassification out of Accumulated Other Comprehensive Income or Loss | Unrecognized Pension and Postretirement Benefit Costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income
Income Before Income Tax	(5.5)	(4.9)
Total Income Tax	(1.2)	(1.7)
Net Income	$ (4.3)	$ (3.2)

Liability for Unpaid Claims a64

Liability for Unpaid Claims and CAE Liability for Unpaid Claims and Claims Adjustment Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense
Balance at January 1	$ 23,222	$ 23,249.5
Less Reinsurance Recoverable	2,182	2,163.6
Net Balance at January 1	21,040	21,085.9
Incurred Related to
Current Year	1,543.1	1,489.8
Total Incurred	1,706.9	1,630.5
Paid Related to
Current Year	(322)	(283.9)
Prior Years	(1,400.4)	(1,369.3)
Total Paid	(1,722.4)	(1,653.2)
Net Balance at March 31	21,024.5	21,063.2
Plus Reinsurance Recoverable	2,184.1	2,139.1
Balance at March 31	23,208.6	23,202.3
Amount Related to Interest
Incurred Related to
Prior Years	283.6	295.7
Incurred Claims
Incurred Related to
Prior Years	(192)	(184.8)
Foreign Currency
Incurred Related to
Prior Years	$ 72.2	$ 29.8

Liability for Unpaid Claims a65

Liability for Unpaid Claims and CAE Reconciliation of Policy and Contract Benefits and Reserves for Future Policy and Contract Benefits (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Policy and Contract Benefits	$ 1,623.1	$ 1,605.2	$ 1,544.8
Reserves for Future Policy and Contract Benefits	44,668.5	45,601.6	44,502.2
Total Policy and Contract Benefits and Reserves for Future Policy and Contract Benefits	46,291.6		46,047
Life Reserves for Future Policy and Contract Benefits	8,278.7		8,105.7
Accident and Health Active Life Reserves	10,774.2		10,271.7
Adjustment Related to Unrealized Investment Gains and Losses	4,030.1		4,467.3
Liability for Unpaid Claims and Claim Adjustment Expense	$ 23,208.6	$ 23,222	$ 23,202.3	$ 23,249.5

Premium Income by Major Line of

Premium Income by Major Line of Business within Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information
Premium Income	$ 2,250	$ 2,142.9
Operating Segments
Segment Reporting Information
Premium Income	2,250	2,142.9
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,438.8	1,360.4
Operating Segments | Unum US | Group Long term Disability
Segment Reporting Information
Premium Income	441.2	434.4
Operating Segments | Unum US | Group Short term Disability
Segment Reporting Information
Premium Income	176.3	157.9
Operating Segments | Unum US | Group Life
Segment Reporting Information
Premium Income	399.2	367.8
Operating Segments | Unum US | Accidental Death Dismemberment
Segment Reporting Information
Premium Income	38.7	36.6
Operating Segments | Unum US | Individual Disability
Segment Reporting Information
Premium Income	104.9	107
Operating Segments | Unum US | Voluntary Benefits
Segment Reporting Information
Premium Income	229.8	215.2
Operating Segments | Unum US | Dental and Vision
Segment Reporting Information
Premium Income	48.7	41.5
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	139.6	121.3
Operating Segments | Unum UK | Group Long term Disability
Segment Reporting Information
Premium Income	90.8	80.8
Operating Segments | Unum UK | Group Life
Segment Reporting Information
Premium Income	28.6	24.8
Operating Segments | Unum UK | Supplemental Insurance
Segment Reporting Information
Premium Income	20.2	15.7
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	398.3	374.3
Operating Segments | Colonial Life | Accident, Sickness, and Disability Insurance
Segment Reporting Information
Premium Income	231.3	219.1
Operating Segments | Colonial Life | Life Insurance Product Line
Segment Reporting Information
Premium Income	81	74.2
Operating Segments | Colonial Life | Cancer and Critical Illness Colonial
Segment Reporting Information
Premium Income	86	81
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	273.3	286.9
Operating Segments | Closed Block | Individual Disability Insurance
Segment Reporting Information
Premium Income	109.4	121.3
Operating Segments | Closed Block | Long-term Care
Segment Reporting Information
Premium Income	161.3	163.1
Operating Segments | Closed Block | Other Insurance Product Line
Segment Reporting Information
Premium Income	$ 2.6	$ 2.5

Selected Operating Statement Da

Selected Operating Statement Data by Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information
Premium Income	$ 2,250	$ 2,142.9
Net Investment Income	602.3	602.4
Other Income	49.5	50.2
Operating Segments
Segment Reporting Information
Premium Income	2,250	2,142.9
Net Investment Income	602.3	602.4
Other Income	49.5	50.2
Adjusted Operating Revenue	2,901.8	2,795.5
Adjusted Operating Income (Loss)	343.3	339.9
Operating Segments | Unum US
Segment Reporting Information
Premium Income	1,438.8	1,360.4
Net Investment Income	194.2	202.5
Other Income	29	28.7
Adjusted Operating Revenue	1,662	1,591.6
Adjusted Operating Income (Loss)	243.9	239.1
Operating Segments | Unum UK
Segment Reporting Information
Premium Income	139.6	121.3
Net Investment Income	27.6	26.6
Other Income	0	0
Adjusted Operating Revenue	167.2	147.9
Adjusted Operating Income (Loss)	29.8	26.6
Operating Segments | Colonial Life
Segment Reporting Information
Premium Income	398.3	374.3
Net Investment Income	37.3	35.1
Other Income	0.3	0.3
Adjusted Operating Revenue	435.9	409.7
Adjusted Operating Income (Loss)	81	82.4
Operating Segments | Closed Block
Segment Reporting Information
Premium Income	273.3	286.9
Net Investment Income	337.7	335.3
Other Income	19	20.8
Adjusted Operating Revenue	630	643
Adjusted Operating Income (Loss)	28.9	31.6
Operating Segments | Corporate and Other
Segment Reporting Information
Premium Income	0	0
Net Investment Income	5.5	2.9
Other Income	1.2	0.4
Adjusted Operating Revenue	6.7	3.3
Adjusted Operating Income (Loss)	$ (40.3)	$ (39.8)

Assets by Segment (Detail)

Assets by Segment (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Assets	$ 63,012.3	$ 64,013.1
Operating Segments
Segment Reporting Information
Assets	63,012.3	64,013.1
Operating Segments | Unum US
Segment Reporting Information
Assets	18,036.3	18,109.1
Operating Segments | Unum UK
Segment Reporting Information
Assets	3,497.6	3,428.1
Operating Segments | Colonial Life
Segment Reporting Information
Assets	4,292.9	4,184.1
Operating Segments | Closed Block
Segment Reporting Information
Assets	34,603.8	35,051.2
Operating Segments | Corporate and Other
Segment Reporting Information
Assets	$ 2,581.7	$ 3,240.6

Reconciliation of Operating Rev

Reconciliation of Operating Revenue and Operating Income by Segment to Revenue and Income Before Tax (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting, Revenue Reconciling Item
Total Revenue	$ 2,899.6	$ 2,806.5
Net Realized Investment Gain (Loss)	(2.2)	11
Income Before Income Tax	341.1	330.3
Net Realized Investment Gain (Loss)	(2.2)	11
Operating Segments
Segment Reporting, Revenue Reconciling Item
Total Revenue	2,899.6	2,806.5
Net Realized Investment Gain (Loss)	(2.2)	11
Adjusted Operating Revenue	2,901.8	2,795.5
Income Before Income Tax	341.1	330.3
Net Realized Investment Gain (Loss)	(2.2)	11
Loss Related to Guaranty Fund Assessment	0	(20.6)
Adjusted Operating Income	$ 343.3	$ 339.9

Segment Information Segment Inf

Segment Information Segment Information - Additional Information (Details)	3 Months Ended
	Mar. 31, 2018Integer
Segment Reporting [Abstract]
Number of Operating Segments	3

Employee Benefit Plans Net Peri

Employee Benefit Plans Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plan | Domestic Plan
Defined Benefit Plan and Other Postretirement Benefit Plans Disclosure
Service Cost	$ 2.3	$ 2
Interest Cost	19.9	21.1
Expected Return on Plan Assets	(26.1)	(25.8)
Amortization of Net Actuarial Loss	5.4	4.9
Amortization of Prior Service Credit	0	(0.1)
Net Periodic Benefit Cost	1.5	2.1
Pension Plan | Foreign Plan
Defined Benefit Plan and Other Postretirement Benefit Plans Disclosure
Service Cost	0	0
Interest Cost	1.5	1.5
Expected Return on Plan Assets	(2.4)	(2)
Amortization of Net Actuarial Loss	0.2	0.2
Amortization of Prior Service Credit	0	0
Net Periodic Benefit Cost	(0.7)	(0.3)
Other Postretirement Benefits Plan
Defined Benefit Plan and Other Postretirement Benefit Plans Disclosure
Service Cost	0	0
Interest Cost	1.2	1.5
Expected Return on Plan Assets	(0.1)	(0.2)
Amortization of Net Actuarial Loss	0	0
Amortization of Prior Service Credit	(0.1)	(0.1)
Net Periodic Benefit Cost	$ 1	$ 1.2

Net Income Per Common Share (De

Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator
Net Income	$ 273.5	$ 229.9
Denominator (000s)
Weighted Average Common Shares - Basic	221,894,000	229,429,600
Dilution for Assumed Exercises of Stock Options and Nonvested Stock Awards	683,000	949,200
Weighted Average Common Shares - Assuming Dilution	222,577,000	230,378,800
Net Income Per Common Share
Basic	$ 1.23	$ 1
Assuming Dilution	$ 1.23	$ 1

Stockholders' Equity and Earn73

Stockholders' Equity and Earnings Per Common Share Treasury Stock Transactions (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Equity, Class of Treasury Stock
Number of Treasury Shares Repurchased	1.9	2.1
Cost of Shares Repurchased	$ 100.2	$ 100
Commission Paid on Common Stock Repurchased	0.2	$ 0
Repurchase Program Authorized in May 2017
Equity, Class of Treasury Stock
Authorized Stock Repurchases	750
Remaining Authorized Share Repurchases	412.8
Repurchase Program Authorized in May 2016
Equity, Class of Treasury Stock
Authorized Stock Repurchases	$ 750

Stockholders' Equity and Earn74

Stockholders' Equity and Earnings Per Common Share - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Stockholders Equity and Earnings Per Common Share Additional Information [Abstract]
Commission Paid on Common Stock Repurchased	$ 0.2	$ 0
Exercise Price Floor on Outstanding Stock Options	$ 23.35
Exercise Price Ceiling on Outstanding Stock Options	26.29
Outstanding Restricted Stock Unit Grant Prices Lower Limit	27.85
Outstanding Restricted Stock Unit Grant Prices Upper Limit	55.26
Outstanding Performance Share Unit Awards Grant Price Range Lower Limit	27.85
Outstanding Performance Share Unit Awards Grant Price Range Upper Limit	$ 49.86
Number of Antidilutive Securities Excluded from Computation of Earnings Per Share		400,000
Authorized Shares of Preferred Stock	25,000,000
Par Value Per Share of Preferred Stock	$ 0.10
Issued Shares of Preferred Stock	0

Debt - Additional Information (

Debt - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)
Debt Narrative Detail Information
Tax Cuts and Jobs Act of 2017, Accounting Complete	false
Letters of Credit Outstanding, Amount	$ 2.1
Amount Outsanding on Credit Facility	0
Northwind Holdings LLC
Debt Narrative Detail Information
Principal Payments on Long-term Debt	$ 15