NMI (NMIH) 10-Q 30 Oct 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 26, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	NMI HOLDINGS, INC.
Entity Central Index Key	1,547,903
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Amendment Flag	false
Entity Filer Category	Accelerated Filer
Entity Emerging Growth Company	true
Entity Small Business	false
Entity Ex Transition Period	true
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Common Stock, Shares Outstanding		66,302,317

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Fixed maturities, available-for-sale, at fair value (amortized cost of $889,794 and $713,859 as of September 30, 2018 and December 31, 2017, respectively)	$ 874,435	$ 715,875
Cash and cash equivalents (including restricted cash of $1,406 and $0 as of September 30, 2018 and December 31, 2017, respectively)	18,187	19,196
Premiums receivable	34,675	25,179
Accrued investment income	5,881	4,212
Prepaid expenses	3,131	2,151
Deferred policy acquisition costs, net	44,437	37,925
Software and equipment, net	22,887	22,802
Intangible assets and goodwill	3,634	3,634
Prepaid reinsurance premiums	33,058	40,250
Deferred tax asset, net	6,880	19,929
Other assets	5,276	3,695
Total assets	1,052,481	894,848
Liabilities
Term loan	147,009	143,882
Unearned premiums	162,893	163,166
Accounts payable and accrued expenses	27,134	23,364
Reserve for insurance claims and claim expenses	10,908	8,761
Reinsurance funds withheld	28,953	34,102
Deferred ceding commission	4,161	5,024
Warrant liability, at fair value	10,930	7,472
Total liabilities	391,988	385,771
Commitments and contingencies
Shareholders' equity
Common stock - class A shares, $0.01 par value; 66,285,847 and 60,517,512 shares issued and outstanding as of September 30, 2018 and December 31, 2017, respectively (250,000,000 shares authorized)	663	605
Additional paid-in capital	678,165	585,488
Accumulated other comprehensive loss, net of tax	(16,303)	(2,859)
Accumulated deficit	(2,032)	(74,157)
Total shareholders' equity	660,493	509,077
Total liabilities and shareholders' equity	$ 1,052,481	$ 894,848

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Fixed maturities, available-for-sale, amortized cost	$ 889,794	$ 713,859
Restricted cash	$ 1,406	$ 0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, issued (in shares)	66,285,847	60,517,512
Common stock, outstanding (in shares)	66,285,847	60,517,512
Common stock, authorized (in shares)	250,000,000	250,000,000

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (UNAUDITED) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues
Net premiums earned	$ 65,407	$ 44,519	$ 181,936	$ 115,661
Net investment income	6,277	4,170	16,586	11,885
Net realized investment (losses) gains	(8)	69	51	198
Other revenues	85	195	193	461
Total revenues	71,761	48,953	198,766	128,205
Expenses
Insurance claims and claim expenses	1,099	957	3,311	2,965
Underwriting and operating expenses	30,379	24,645	87,852	78,682
Total expenses	31,478	25,602	91,163	81,647
Other expense
Loss from change in fair value of warrant liability	(5,464)	(502)	(4,935)	(679)
Interest expense	(2,972)	(3,352)	(11,951)	(10,146)
Total other expense	(8,436)	(3,854)	(16,886)	(10,825)
Income before income taxes	31,847	19,497	90,717	35,733
Income tax expense	7,036	7,185	18,310	11,917
Net income	$ 24,811	$ 12,312	$ 72,407	$ 23,816
Earnings per share
Basic earnings per share (in dollars per share)	$ 0.38	$ 0.21	$ 1.12	$ 0.40
Diluted earnings per share (in dollars per share)	$ 0.36	$ 0.20	$ 1.07	$ 0.38
Weighted average common shares outstanding
Basic weighted average shares outstanding (in shares)	65,948	59,884	64,584	59,680
Dilutive weighted average shares outstanding (in shares)	68,844	63,089	67,512	62,773
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) in accumulated other comprehensive income, net of tax (benefit) expense of ($337) and $366 for the three months ended September 30, 2018 and 2017, respectively, and ($3,676) and $2,439 for the nine months ended September 30, 2018 and 2017	$ (1,267)	$ 768	$ (13,828)	$ 4,786
Reclassification adjustment for realized losses (gains) included in net income, net of tax expense (benefit) of ($2) and $24 for the three months ended September 30, 2018 and 2017, respectively, and ($27) and $69 for the nine months ended September 30, 2018 and 2017	7	(45)	102	(129)
Other comprehensive (loss) income, net of tax	(1,260)	723	(13,726)	4,657
Comprehensive income	$ 23,551	$ 13,035	$ 58,681	$ 28,473

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Net unrealized (losses) gains in accumulated other comprehensive income, tax (benefit) expense	$ (337)	$ 366	$ (3,676)	$ 2,439
Reclassification adjustment for realized losses included in net income, tax expense	$ (2)	$ 24	$ (27)	$ 69

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands		Total	Common Stock - Class A		Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle		$ 515					$ 515
Beginning balance at Dec. 31, 2016		475,509	$ 591		$ 576,927	$ (5,287)	(96,722)
Beginning balance (in shares) at Dec. 31, 2016			59,145,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes		(1,110)	$ 7		(1,117)
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes (in shares)			638,000
Share-based compensation expense		2,271			2,271
Change in unrealized investment gains/losses, net of tax		1,233				1,233
Net income		5,492					5,492
Ending balance at Mar. 31, 2017		483,910	$ 598		578,081	(4,054)	(90,715)
Ending balance (in shares) at Mar. 31, 2017			59,783,000
Beginning balance at Dec. 31, 2016		475,509	$ 591		576,927	(5,287)	(96,722)
Beginning balance (in shares) at Dec. 31, 2016			59,145,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income		23,816
Ending balance at Sep. 30, 2017		511,025	$ 599		583,447	(630)	(72,391)
Ending balance (in shares) at Sep. 30, 2017			59,928,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle		388			388
Beginning balance at Mar. 31, 2017		483,910	$ 598		578,081	(4,054)	(90,715)
Beginning balance (in shares) at Mar. 31, 2017			59,783,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes		82			82
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes (in shares)			75,000
Share-based compensation expense		1,948			1,948
Change in unrealized investment gains/losses, net of tax		2,700				2,700
Net income		6,012					6,012
Ending balance at Jun. 30, 2017		495,040	$ 598		580,499	(1,354)	(84,703)
Ending balance (in shares) at Jun. 30, 2017			59,858,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes		235	$ 1		234
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes (in shares)			70,000
Share-based compensation expense		2,714			2,714
Change in unrealized investment gains/losses, net of tax		724				724
Net income		12,312					12,312
Ending balance at Sep. 30, 2017		511,025	$ 599		583,447	(630)	(72,391)
Ending balance (in shares) at Sep. 30, 2017			59,928,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle		0				282	(282)
Beginning balance at Dec. 31, 2017		509,077	$ 605		585,488	(2,859)	(74,157)
Beginning balance (in shares) at Dec. 31, 2017			60,518,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock: class A shares issued related to public offering		79,165	$ 43		79,122
Common stock: class A shares issued related to public offering (in shares)			4,255,000
Common stock: class A shares issued related to warrants	[1]	$ 489			489
Common stock: class A shares issued related to warrants (in shares)		25,686	26,000	[1]
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes		$ (991)	$ 8		(999)
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes (in shares)			770,000
Share-based compensation expense		2,805			2,805
Change in unrealized investment gains/losses, net of tax		(10,956)				(10,956)
Net income		22,355					22,355
Ending balance at Mar. 31, 2018		601,944	$ 656		666,905	(13,533)	(52,084)
Ending balance (in shares) at Mar. 31, 2018			65,569,000
Beginning balance at Dec. 31, 2017		$ 509,077	$ 605		585,488	(2,859)	(74,157)
Beginning balance (in shares) at Dec. 31, 2017			60,518,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock: class A shares issued related to warrants (in shares)		87,000
Net income		$ 72,407
Ending balance at Sep. 30, 2018		660,493	$ 663		678,165	(16,303)	(2,032)
Ending balance (in shares) at Sep. 30, 2018			66,286,000
Beginning balance at Mar. 31, 2018		601,944	$ 656		666,905	(13,533)	(52,084)
Beginning balance (in shares) at Mar. 31, 2018			65,569,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock: class A shares issued related to warrants	[1]	$ 63			63
Common stock: class A shares issued related to warrants (in shares)		3,751	3,000	[1]
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes		$ 887	$ 2		885
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes (in shares)			182,000
Share-based compensation expense		3,017			3,017
Change in unrealized investment gains/losses, net of tax		(1,510)				(1,510)
Net income		25,241					25,241
Ending balance at Jun. 30, 2018		629,642	$ 658		670,870	(15,043)	(26,843)
Ending balance (in shares) at Jun. 30, 2018			65,754,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock: class A shares issued related to warrants		1,245	$ 1		1,244
Common stock: class A shares issued related to warrants (in shares)			57,000
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes		3,096	$ 4		3,092
Common stock: class A shares issued under stock plans, net of shares withheld for employee taxes (in shares)			475,000
Share-based compensation expense		2,959			2,959
Change in unrealized investment gains/losses, net of tax		(1,260)				(1,260)
Net income		24,811					24,811
Ending balance at Sep. 30, 2018		$ 660,493	$ 663		$ 678,165	$ (16,303)	$ (2,032)
Ending balance (in shares) at Sep. 30, 2018			66,286,000
[1]	During the three month periods ended March 31, 2018 and June 30, 2018, we issued 25,686 and 3,751 common shares, respectively, with a par value of $0.01 related to the exercise of warrants, which is not identifiable in this schedule due to rounding.

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Statement of Stockholders' Equity [Abstract]
Change in unrealized investment gains/losses, tax expense (benefit)	$ (335)	$ (2,891)	$ (423)	$ (390)	$ (1,454)	$ 664
Stock issued upon exercise of warrants (in shares)		3,751	25,686
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01

CONDENSED CONSOLIDATED STATEM_5

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities
Net income		$ 72,407	$ 23,816
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Net realized investment gains		(51)	(198)
Loss from change in fair value of warrant liability		4,935	679
Depreciation and amortization		5,825	4,871
Net amortization of premium on investment securities		1,176	1,200
Amortization of debt discount and debt issuance costs		3,141	1,112
Share-based compensation expense		8,781	6,933
Deferred income taxes		16,698	11,340
Changes in operating assets and liabilities:
Premiums receivable		(9,496)	(7,328)
Accrued investment income		(1,669)	(1,177)
Prepaid expenses		(980)	(660)
Deferred policy acquisition costs, net		(6,512)	(5,992)
Other assets	[1]	927	(273)
Unearned premiums		(273)	8,439
Reserve for insurance claims and claim expenses		2,147	3,122
Reinsurance balances, net	[1]	565	(259)
Accounts payable and accrued expenses		1,728	(3,847)
Net cash provided by operating activities		99,349	41,778
Cash flows from investing activities
Purchase of short-term investments		(168,751)	(111,551)
Purchase of fixed-maturity investments, available-for-sale		(310,286)	(166,640)
Proceeds from maturity of short-term investments		148,997	142,722
Proceeds from redemptions, maturities and sale of fixed-maturity investments, available-for-sale		154,438	75,785
Additions to software and equipment		(5,326)	(6,869)
Net cash used in investing activities		(180,928)	(66,553)
Cash flows from financing activities
Proceeds from issuance of common stock related to public offering, net of issuance costs		79,165	0
Proceeds from issuance of common stock related to employee equity plans		12,557	3,105
Proceeds from issuance of common stock related to warrants		320	0
Taxes paid related to net share settlement of equity awards		(10,113)	(3,883)
Proceeds from senior note, net		149,250	0
Repayments of term loan		(147,000)	(1,125)
Payments of debt issuance/modification costs		(3,609)	(370)
Net cash provided by (used in) financing activities		80,570	(2,273)
Net decrease in cash, cash equivalents and restricted cash		(1,009)	(27,048)
Cash, cash equivalents and restricted cash, beginning of period		19,196	47,746
Cash, cash equivalents and restricted cash, end of period		18,187	20,698
Supplemental disclosures of cash flow information
Interest paid		9,233	10,350
Income tax paid		$ 687	$ 802
[1]	Ceded losses recoverable on the QSR Transactions were reclassified from "Other Assets" in prior periods to "Reinsurance balance, net".

Organization, Basis of Presenta

Organization, Basis of Presentation and Summary of Accounting Principles	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization, Basis of Presentation and Summary of Accounting Principles	Organization, Basis of Presentation and Summary of Accounting Principles NMI Holdings, Inc. (NMIH) is a Delaware corporation, incorporated in May 2011, to provide private mortgage guaranty insurance (which we refer to as mortgage insurance or MI) through its wholly owned insurance subsidiaries, National Mortgage Insurance Corporation (NMIC) and National Mortgage Reinsurance Inc One (Re One). In April 2012, we completed a private placement of our securities, through which we offered and sold an aggregate of 55 million of our Class A common stock resulting in net proceeds of approximately $510 million (the Private Placement), and we completed the acquisition of our insurance subsidiaries for $8.5 million in cash, common stock and warrants, plus the assumption of $1.3 million in liabilities. In November 2013, we completed an initial public offering of 2.4 million shares of our common stock, and our common stock began trading on the NASDAQ exchange on November 8, 2013, under the symbol "NMIH." In March 2018, we completed the sale of an additional 4.3 million shares of common stock including a 15% option to purchase additional shares, which was exercised in full. In April 2013, NMIC, our primary insurance subsidiary, issued its first mortgage insurance policy. NMIC is licensed to write mortgage insurance in all 50 states and D.C. In August 2015, NMIH capitalized a wholly owned subsidiary, NMI Services, Inc. (NMIS), through which we offer outsourced loan review services to mortgage loan originators. Basis of Presentation The accompanying unaudited condensed consolidated financial statements, which include the results of NMIH and its wholly owned subsidiaries, have been prepared in accordance with the instructions to Form 10-Q as prescribed by the SEC for interim reporting and include other information and disclosures required by accounting principles generally accepted in the U.S. (GAAP). Our accounts are maintained in U.S. dollars. These statements should be read in conjunction with our consolidated financial statements and notes thereto for the year ended December 31, 2017 , included in our 2017 10-K. All intercompany transactions have been eliminated. The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect reported amounts of assets and liabilities, as well as disclosure of contingent assets and liabilities as of the balance sheet date. Estimates also affect the reported amounts of income and expenses for the reporting period. Actual results could differ from those estimates. Certain reclassifications to our previously reported financial information have been made to conform to current period presentation. The results of operations for the interim period may not be indicative of the results that may be expected for the full year ending December 31, 2018 . Significant Accounting Principles There have been no changes to our significant accounting principles as described in Item 8, " Financial Statements and Supplementary Data - Notes to Consolidated Financial Statements - Note 2 - Summary of Accounting Principles " of our 2017 Form 10-K, other than as noted in " Reinsurance ", " Variable interest entity" and " Recent Accounting Pronouncements - Adopted" below. Reinsurance We account for premiums, claims and claim expenses that are ceded to reinsurers on a basis consistent with those we use to account for the original policies we issue and pursuant to the terms of our reinsurance contracts. We account for premiums ceded or otherwise paid to reinsurers as reductions to premium revenue. Effective January 1, 2018, NMIC entered into a second quota share reinsurance transaction (2018 QSR Transaction) which is similar in nature to the quota share reinsurance transaction we entered into in September 2016 (2016 QSR Transaction, together with 2018 QSR Transaction, the QSR Transactions) (see Note 5, " Reinsurance "). We earn profit and ceding commissions in connection with the QSR Transactions. Profit commissions represent a percentage of the profits recognized by reinsurers that are returned to us, based on the level of claims and claim expenses that we cede. We recognize any profit commissions we earn as increases to premium revenue. Ceding commissions are calculated as a percentage of ceded written premiums under the 2016 QSR Transaction and as a percentage of ceded earned premiums under the 2018 QSR Transaction, to cover our costs to acquire and service the direct policies. We earn the ceding commissions in a manner consistent with our recognition of earnings on the underlying insurance policies, over the terms of the policies reinsured. We account for ceding commissions earned as a reduction to underwriting and operating expenses. Under the QSR Transactions, we cede a portion of claims and claim expenses reserves to our reinsurers, which are accounted for as reinsurance recoverables in "Other Assets" on the consolidated balance sheets and as reductions to claim expense on the consolidated statements of operations. We remain directly liable for all loss payments in the event we are unable to collect from any reinsurer. Variable interest entity NMIC entered into aggregate excess of loss reinsurance agreements with Oaktown Re Ltd. (Oaktown Re) and Oaktown Re II Ltd (Oaktown Re II), each a Bermuda-domiciled special purpose reinsurer, in May 2017 and August 2018, respectively. At inception of each reinsurance agreement, we determined that each of Oaktown Re and Oaktown Re II were variable interest entities (VIEs), as defined under GAAP (ASC 810), because they did not have sufficient equity at risk to finance their respective activities. We evaluated the VIEs at inception to determine whether NMIC was the primary beneficiary under each deal and, if so, whether we were required to consolidate the assets and liabilities of each VIE. The primary beneficiary of a VIE is an enterprise that (1) has the power to direct the activities of the VIE, which most significantly impact its economic performance and (2) has significant economic exposure to the VIE; i.e., the obligation to absorb losses or receive benefits that could potentially be significant. The determination of whether an entity is the primary beneficiary of a VIE is complex and requires management judgment regarding determinative factors, including the expected results of the VIE and how those results are absorbed by beneficial interest holders, as well as which party has the power to direct activities that most significantly impact the performance of the VIE. We concluded that we are not the primary beneficiary of either Oaktown Re or Oaktown Re II because NMIC does not have significant economic exposure to either Oaktown Re or Oaktown Re II, and, as such, we do not consolidate the VIEs in our consolidated financial statements. Recent Accounting Pronouncements - Adopted In May 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2014-09, Revenue from Contracts with Customers (Topic 606). This update is intended to provide a consistent approach in recognizing revenue. In December 2016, the FASB clarified that all contracts that are within the scope of Topic 944, Financial Services-Insurance , are excluded from the scope of ASU 2014-09. Accordingly, this update did not impact the recognition of revenue related to insurance premiums or investment income, which represent a majority of our total revenues. The update impacted our loan review services revenue, which is the only revenue stream in scope of the update. We adopted this update on January 1, 2018 using the modified-retrospective approach and the impact was immaterial to our consolidated financial statements. In January 2016, the FASB issued ASU 2016-01, Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Financial Liabilities (Subtopic 825-10). This update requires entities to reduce the carrying amount of deferred tax assets, if necessary, by the amount of any tax benefit that is not expected to be realized. We adopted this update effective January 1, 2018. The impact was immaterial to our consolidated financial statements. In February 2018, the FASB issued ASU 2018-02, Reclassification of Ce rtain Tax Effects from Accumulated Other Comprehensive Income (Topic 220). This update permits a company to reclassify the disproportionate income tax effects as a result of the 2017 Tax Cuts and Jobs Act (the TCJA) on items within accumulated other comprehensive income (AOCI) to retained earnings. This standard will take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted. We adopted this update on January 1, 2018 and adjusted the disproportionate incom e tax effects, or "stranded tax effects," resulting in a $0.3 million reduction to our beginning retained earnings as of January 1, 2018. Recent Accounting Pronouncements - Not Yet Adopted In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). This update requires that businesses recognize rights and obligations associated with certain leases as assets and liabilities on the balance sheet. The standard also requires additional disclosures regarding the amount, timing, and uncertainty of cash flows arising from leases. For public business entities, this update is effective for annual periods beginning after December 15, 2018 and interim periods therein. Early adoption is permitted in any period. We expect to adopt this guidance on January 1, 2019. In September 2017, ASU 2017-13 added guidance from a SEC Staff Announcement, "Transition Related to Accounting Standards Update No. 2016-02," and in July 2018, the FASB issued ASU 2018-11, Leases (Topic 842), Targeted Improvement, which provides companies the option to apply the provisions of the new lease standard prospectively as of the effective date, without adjusting comparative periods presented. The effect of adoption will depend on our current lease portfolio at time of adoption; however, upon adoption, we anticipate that our reported assets and liabilities will increase in connection with the recognition of any right-of-use assets and lease liabilities, such as the operating lease on our corporate headquarters. In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326). This update requires companies to measure all expected credit losses for financial assets held at the reporting date. The standard also amends the accounting for credit losses on available-for-sale debt securities and purchased financial assets with credit deterioration. The standard will take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. We are currently reviewing the impacts the adoption of this guidance will have, if any, on our accounting for credit losses on our investment portfolio. We do not expect it to impact our accounting for insurance claims and claims expenses as these items are not in the scope of this ASU. In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), and Derivatives and Hedging (Topic 815). This update is intended to simplify the accounting for certain equity-linked financial instruments. The standard will take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted. The guidance must be applied using a full or modified retrospective approach. We expect to adopt this ASU in the first quarter of 2019 and have determined that it will have no impact on our consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Compensation-Stock Compensation (Topic 718). This update expands the scope of Topic 718 to include share-based payments made to non-employees in connection with the acquisition of goods and services. The standard will take effect for public business entities for fiscal years, beginning after December 15, 2019, and interim periods within fiscal years, beginning after December 15, 2020. We expect to adopt this ASU in the first quarter of 2019 and have determined that it has no impact on our financial results at this time as we have not made any share-based grants to nonemployees as defined in ASC 718-10-20. In August 2018, the FASB issued ASU 2018-12, Targeted Improvements to the Accounting for Long-Duration Contracts . This update provides guidance to the existing recognition, measurement, presentation and disclosure requirements for long-duration contracts issued by an insurance entity. The standard will take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020. We are currently evaluating the impact the adoption of this ASU will have, if any, on our consolidated financial statements. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820). This update requires companies to make disclosures about recurring and nonrecurring fair value measurements. The standard will take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. We are currently evaluating the impact the adoption of this ASU will have, if any, on our fair value of financial instruments disclosures. In August 2018, the FASB issued ASU 2018 -15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40). This update applies to cloud computing arrangements hosted by a vendor and provides companies with guidance on the criteria for capitalizing implementation, set-up and other up-front costs incurred in association with these arrangements. The standard will take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. We are currently evaluating the impact the adoption of this ASU will have, if any, on our consolidated financial statements.

Investments

Investments	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Investments	Investments We have designated our investment portfolio as available-for-sale and report it at fair value. The related unrealized gains and losses are, after considering the related tax expense or benefit, recognized through comprehensive income and loss, and on an accumulated basis in shareholders' equity. Net realized investment gains and losses are reported in income based upon specific identification of securities sold. Fair Values and Gross Unrealized Gains and Losses on Investments Amortized Gross Unrealized Fair Gains Losses As of September 30, 2018 (In Thousands) U.S. Treasury securities and obligations of U.S. government agencies $ 48,292 $ — $ (2,483 ) $ 45,809 Municipal debt securities 93,945 20 (1,933 ) 92,032 Corporate debt securities 536,240 323 (10,217 ) 526,346 Asset-backed securities 169,061 113 (1,236 ) 167,938 Total bonds 847,538 456 (15,869 ) 832,125 Short-term investments 42,256 54 — 42,310 Total investments $ 889,794 $ 510 $ (15,869 ) $ 874,435 Amortized Gross Unrealized Fair Gains Losses As of December 31, 2017 (In Thousands) U.S. Treasury securities and obligations of U.S. government agencies $ 65,669 $ — $ (981 ) $ 64,688 Municipal debt securities 89,973 534 (659 ) 89,848 Corporate debt securities 435,562 4,231 (1,958 ) 437,835 Asset-backed securities 100,153 916 (125 ) 100,944 Total bonds 691,357 5,681 (3,723 ) 693,315 Long-term investments - other 353 — — 353 Short-term investments 22,149 58 — 22,207 Total investments $ 713,859 $ 5,739 $ (3,723 ) $ 715,875 As of September 30, 2018 and December 31, 2017 , approximately $5.3 million and $7.0 million , respectively, of our cash and investments were held in the form of U.S. Treasury securities on deposit with various state insurance departments to satisfy regulatory requirements. See Note 5 "Reinsurance " for the information related to restricted cash. Scheduled Maturities The amortized cost and fair values of available-for-sale securities as of September 30, 2018 and December 31, 2017 , by contractual maturity, are shown below. Expected maturities will differ from contractual maturities because issuers may have the right to call or prepay obligations with or without call or prepayment penalties. Because most asset-backed securities provide for periodic payments throughout their lives, they are listed below in a separate category. As of September 30, 2018 Amortized Fair (In Thousands) Due in one year or less $ 63,314 $ 63,368 Due after one through five years 335,262 330,603 Due after five through ten years 318,357 308,759 Due after ten years 3,800 3,767 Asset-backed securities 169,061 167,938 Total investments $ 889,794 $ 874,435 As of December 31, 2017 Amortized Fair (In Thousands) Due in one year or less $ 97,406 $ 97,394 Due after one through five years 195,795 195,626 Due after five through ten years 305,798 306,930 Due after ten years 14,707 14,981 Asset-backed securities 100,153 100,944 Total investments $ 713,859 $ 715,875 Aging of Unrealized Losses As of September 30, 2018 , the investment portfolio had gross unrealized losses of $15.9 million , $7.8 million of which has been in an unrealized loss position for a period of 12 months or greater. We did not consider these securities to be other-than-temporarily impaired as of September 30, 2018 . We based our conclusion that these investments were not other-than-temporarily impaired as of September 30, 2018 on the following facts: (i) the unrealized losses were primarily caused by interest rate movements since the purchase date; (ii) we do not intend to sell these investments; and (iii) we do not believe that it is more likely than not that we will be required to sell these investments before recovery of our amortized cost basis, which may not occur until maturity. For those securities in an unrealized loss position, the length of time the securities were in such a position is as follows: Less Than 12 Months 12 Months or Greater Total # of Securities Fair Value Unrealized Losses # of Securities Fair Value Unrealized Losses # of Securities Fair Value Unrealized Losses As of September 30, 2018 (Dollars in Thousands) U.S. Treasury securities and obligations of U.S. government agencies 5 $ 5,033 $ (60 ) 18 $ 40,776 $ (2,423 ) 23 $ 45,809 $ (2,483 ) Municipal debt securities 26 53,431 (885 ) 18 30,580 (1,048 ) 44 84,011 (1,933 ) Corporate debt securities 180 343,602 (6,123 ) 43 76,589 (4,094 ) 223 420,191 (10,217 ) Asset-backed securities 51 111,382 (976 ) 6 10,563 (260 ) 57 121,945 (1,236 ) Total 262 $ 513,448 $ (8,044 ) 85 $ 158,508 $ (7,825 ) 347 $ 671,956 $ (15,869 ) Less Than 12 Months 12 Months or Greater Total # of Securities Fair Value Unrealized Losses # of Securities Fair Value Unrealized Losses # of Securities Fair Value Unrealized Losses As of December 31, 2017 (Dollars in Thousands) U.S. Treasury securities and obligations of U.S. government agencies 16 $ 29,806 $ (394 ) 26 $ 34,882 $ (587 ) 42 $ 64,688 $ (981 ) Municipal debt securities 21 38,628 (264 ) 10 17,945 (395 ) 31 56,573 (659 ) Corporate debt securities 94 128,313 (829 ) 23 48,978 (1,129 ) 117 177,291 (1,958 ) Asset-backed securities 22 27,947 (63 ) 5 12,438 (62 ) 27 40,385 (125 ) Total 153 $ 224,694 $ (1,550 ) 64 $ 114,243 $ (2,173 ) 217 $ 338,937 $ (3,723 ) The following table presents the components of net investment income: For the three months ended September 30, For the nine months ended September 30, 2018 2017 2018 2017 (In Thousands) Investment income $ 6,473 $ 4,363 $ 17,192 $ 12,455 Investment expenses (196 ) (193 ) (606 ) (570 ) Net investment income $ 6,277 $ 4,170 $ 16,586 $ 11,885 The following table presents the components of net realized investment gains (losses): For the three months ended September 30, For the nine months ended September 30, 2018 2017 2018 2017 (In Thousands) Gross realized investment gains $ 461 $ 69 $ 520 $ 536 Gross realized investment losses (469 ) — (469 ) (338 ) Net realized investment gains (losses) $ (8 ) $ 69 $ 51 $ 198 Investment Securities - Other-than-Temporary Impairment (OTTI) At September 30, 2018 , we held no other-than-temporarily impaired securities. During the three and nine months ended September 30, 2018 and the three months ended September 30, 2017, we did not recognize any OTTI losses. During the nine months ended September 30, 2017, we recognized $0.1 million of OTTI losses in earnings.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	Fair Value of Financial Instruments The following describes the valuation techniques used by us to determine the fair value of our financial instruments: We established a fair value hierarchy by prioritizing the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements) and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy under this standard are described below: Level 1 - Fair value measurements based on quoted prices in active markets that we have the ability to access for identical assets or liabilities. Market price data generally is obtained from exchange or dealer markets. We do not adjust the quoted price for such instruments. Level 2 - Fair value measurements based on inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. Level 2 inputs include quoted prices for similar assets and liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and inputs other than quoted prices that are observable for the asset or liability, such as interest rates and yield curves that are observable at commonly quoted intervals. Level 3 - Fair value measurements based on valuation techniques that use significant inputs that are unobservable. Both observable and unobservable inputs may be used to determine the fair values of positions classified in Level 3. The circumstances for using these measurements include those in which there is little, if any, market activity for the asset or liability. Therefore, we must make certain assumptions, which require significant management judgment or estimation about the inputs a hypothetical market participant would use to value that asset or liability. In certain cases, the inputs used to measure fair value may fall into different levels of the fair value hierarchy. In such cases, the level in the fair value hierarchy is determined based on the lowest level input that is significant to the fair value measurement in its entirety. Assets classified as Level 1 and Level 2 To determine the fair value of securities available-for-sale in Level 1 and Level 2 of the fair value hierarchy, independent pricing sources have been utilized. One price is provided per security based on observable market data. To ensure securities are appropriately classified in the fair value hierarchy, we review the pricing techniques and methodologies of the independent pricing sources and believe that their policies adequately consider market activity, either based on specific transactions for the issue valued or based on modeling of securities with similar credit quality, duration, yield and structure that were recently traded. A variety of inputs are utilized by the independent pricing sources including benchmark yields, reported trades, non-binding broker/dealer quotes, issuer spreads, two sided markets, benchmark securities, bids, offers and reference data including data published in market research publications. Inputs may be weighted differently for any security, and not all inputs are used for each security evaluation. Market indicators, industry and economic events are also considered. This information is evaluated using a multidimensional pricing model. Quality controls are performed by the independent pricing sources throughout this process, which include reviewing tolerance reports, trading information and data changes, and directional moves compared to market moves. This model combines all inputs to arrive at a value assigned to each security. We have not made any adjustments to the prices obtained from the independent pricing sources. Liabilities classified as Level 3 We calculate the fair value of outstanding warrants utilizing Level 3 inputs, including a Black-Scholes option-pricing model, in combination with a binomial model, and we value the pricing protection features within the warrants using a Monte-Carlo simulation model. Variables in the model include the risk-free rate of return, dividend yield, expected life and expected volatility of our stock price. The following tables present the level within the fair value hierarchy at which our financial instruments were measured: Fair Value Measurements Using Quoted Prices in Significant Other Significant Fair Value As of September 30, 2018 (In Thousands) U.S. Treasury securities and obligations of U.S. government agencies $ 45,809 $ — $ — $ 45,809 Municipal debt securities — 92,032 — 92,032 Corporate debt securities — 526,346 — 526,346 Asset-backed securities — 167,938 — 167,938 Long-term investment – other — — — Cash, cash equivalents and short-term investments 60,497 60,497 Total assets $ 106,306 $ 786,316 $ — $ 892,622 Warrant liability — — 10,930 10,930 Total liabilities $ — $ — $ 10,930 $ 10,930 Fair Value Measurements Using Quoted Prices in Significant Other Significant Fair Value As of December 31, 2017 (In Thousands) U.S. Treasury securities and obligations of U.S. government agencies $ 59,844 $ 4,844 $ — $ 64,688 Municipal debt securities — 89,848 — 89,848 Corporate debt securities — 437,835 — 437,835 Asset-backed securities — 100,944 — 100,944 Long-term investment - other 353 — — 353 Cash, cash equivalents and short-term investments 41,403 — — 41,403 Total assets $ 101,600 $ 633,471 $ — $ 735,071 Warrant liability — — 7,472 7,472 Total liabilities $ — $ — $ 7,472 $ 7,472 There were no transfers between Level 1 and Level 2, nor any transfers in or out of Level 3, of the fair value hierarchy during the nine months ended September 30, 2018 and the year ended December 31, 2017 . The following is a roll-forward of Level 3 liabilities measured at fair value: For the nine months ended September 30, Warrant Liability 2018 2017 (In Thousands) Balance, January 1 $ 7,472 $ 3,367 Change in fair value of warrant liability included in earnings 4,935 679 Issuance of common stock on warrant exercise (1,477 ) — Balance, September 30 $ 10,930 $ 4,046 The following table outlines the key inputs and assumptions used to calculate the fair value of the warrant liability in the Black-Scholes option-pricing model as of the dates indicated. As of September 30, 2018 2017 Common stock price $ 22.65 $ 12.40 Risk free interest rate 2.86 - 2.90% 1.66 % Expected life 2.50 - 3.56 Years 3.25 years Expected volatility 39.9 - 41.5% 30.6 % Dividend yield 0% 0% The changes in fair value of the warrant liability for the nine months ended September 30, 2018 and 2017 are primarily attributable to changes in the price of our common stock during the respective periods, with additional impact related to changes in the Black-Scholes model inputs and exercises of outstanding warrants.

Debt Obligations

Debt Obligations	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Debt Obligations	Debt Obligations On May 24, 2018, we entered into a credit agreement (2018 Credit Agreement), which provides for (i) a $150 million five-year senior secured term loan facility (2018 Term Loan) that matures on May 24, 2023; and (ii) a $85 million three-year secured revolving credit facility (2018 Revolving Credit Facility) that matures on May 24, 2021. Proceeds from the 2018 Term Loan were used to repay in full the outstanding amount due under our $150 million amended term loan due November 10, 2019 (the 2015 Term Loan), and to pay fees and expenses incurred in connection with the 2018 Credit Agreement. 2018 Term Loan The 2018 Term Loan bears interest at the Eurodollar Rate, as defined in the 2018 Credit Agreement and subject to a 1.00% floor, plus an annual margin rate of 4.75% , representing an all-in rate of 6.99% as of September 30, 2018 , payable monthly based on our current interest period election. Quarterly principal payments of $375 thousand are also required. The 2018 Term Loan has a prepayment premium of 1% for any refinancing prepayments made on or prior to the date that is six months after the date of the 2018 Credit Agreement, after which there is no prepayment penalty. Interest expense for nine months ended September 30, 2018 includes amounts related to our interest payment, one-time financing costs and the amortization of issuance costs and original issue discounts. For the nine months ended September 30, 2018 , we recorded $11.6 million of interest expense, including $2.2 million of costs related to the extinguishment of the 2015 Term Loan and issuance of the 2018 Term Loan. Capitalized debt issuance costs totaling $1.8 million and original issue discounts totaling $986 thousand are being amortized to interest expense using the effective interest method over the contractual life of the 2018 Term Loan. As of September 30, 2018 , the remaining unamortized issuance cost and original issue discounts totaled $2.6 million , and the outstanding principal balance of the 2018 Term Loan was $149.6 million . We are subject to certain covenants under the 2018 Term Loan (as defined in the 2018 Credit Agreement), including (but not limited to) a maximum debt-to-total capitalization ratio (as defined in the 2018 Credit Agreement) of 35% under the 2018 Term Loan. We were in compliance with all covenants as of September 30, 2018 . Future principal payments due under the 2018 Term Loan as of September 30, 2018 are as follows: As of September 30, 2018 Principal (In thousands) 2018 $ 375 2019 1,500 2020 1,500 2021 1,500 2022 1,500 2023 143,250 Total $ 149,625 2018 Revolving Credit Facility Borrowings under the 2018 Revolving Credit Facility may be used for general corporate purposes and will accrue interest at a variable rate equal to, at our discretion, (i) a base rate (as defined in the 2018 Credit Agreement, subject to a floor of 1.00% per annum) plus a margin of 1.00% to 2.50% per annum, based on the applicable corporate credit rating at the time, or (ii) the Eurodollar Rate (subject to a floor of 0.00% per annum) plus a margin of 2.00% to 3.50% per annum, based on the applicable corporate credit rating at the time. As of September 30, 2018 , no borrowings had been made under the 2018 Revolving Credit Facility. We are required to pay a quarterly commitment fee on the average daily undrawn amount of the 2018 Revolving Credit Facility, which ranges from 0.30% to 0.60% , based on the applicable corporate credit rating at the time. As of September 30, 2018 , the applicable commitment fee was 0.50% . For the three and nine months ended September 30, 2018, we recorded $0.1 million and $0.2 million of commitment fees in interest expense, respectively. We incurred issuance costs of $1.5 million in connection with the establishment of the 2018 Revolving Credit Facility, which were deferred and recorded within Other Assets. These costs will be amortized through interest expense over the three-year life of the 2018 Revolving Credit Facility on a straight line basis. For the three and nine months ended September 30, 2018 , we recognized $0.1 million and $0.2 million of interest expense from the amortization of deferred issuance costs. At September 30, 2018, the remaining issuance costs were $1.3 million , net of accumulated amortization. We are subject to certain covenants under the 2018 Revolving Credit Facility, including (but not limited to) the following: a maximum debt-to-total capitalization ratio of 35% , a minimum liquidity requirement, compliance with the PMIERs financial requirements (subject to any GSE-approved waivers), and minimum consolidated net worth and statutory capital requirements (respectively, as defined therein). We were in compliance with all covenants as of September 30, 2018 .

Reinsurance

Reinsurance	9 Months Ended
	Sep. 30, 2018
Reinsurance Disclosures [Abstract]
Reinsurance	Reinsurance We enter into third-party reinsurance transactions to actively manage our risk, ensure PMIERs compliance and support the growth of our business. The GSEs and the Wisconsin Office of the Commissioner of Insurance (Wisconsin OCI) have approved all such transactions (subject to certain conditions and ongoing review, including levels of approved capital credit). The effect of our reinsurance agreements on premiums written and earned is as follows: For the three months ended For the nine months ended September 30, 2018 September 30, 2017 September 30, 2018 September 30, 2017 (In Thousands) Net premiums written Direct $ 73,748 $ 56,217 $ 210,452 $ 142,134 Ceded (1) (8,367 ) (8,501 ) (21,598 ) (20,029 ) Net premiums written $ 65,381 $ 47,716 $ 188,854 $ 122,105 Net premiums earned Direct $ 76,513 $ 52,024 $ 210,725 $ 133,696 Ceded (1) (11,106 ) (7,505 ) (28,789 ) (18,035 ) Net premiums earned $ 65,407 $ 44,519 $ 181,936 $ 115,661 (1) Net of profit commission Excess-of-loss reinsurance 2017 ILN Transaction In May 2017, NMIC entered into a reinsurance agreement with Oaktown Re that provides for up to $211.3 million of aggregate excess-of-loss reinsurance coverage at inception for new delinquencies on an existing portfolio of mortgage insurance policies written from 2013 through December 31, 2016. For the reinsurance coverage period, NMIC retains the first layer of $126.8 million of aggregate losses, of which $125.6 million remained at September 30, 2018 , and Oaktown Re then provides second layer coverage up to the outstanding reinsurance coverage amount. NMIC will then retain losses in excess of the outstanding reinsurance coverage amount. The outstanding reinsurance coverage amount decreases from $211.3 million at inception over a ten -year period as the underlying covered mortgages are amortized or repaid, and/or the mortgage insurance coverage is canceled and was $144.1 million as of September 30, 2018 . The outstanding reinsurance coverage amount will stop amortizing if certain credit enhancement or delinquency thresholds are triggered. Oaktown Re financed the coverage by issuing mortgage insurance-linked notes in an aggregate amount of $211.3 million to unaffiliated investors (the 2017 Notes). The 2017 Notes mature on April 26, 2027. All of the proceeds paid to Oaktown Re from the sale of the 2017 Notes were deposited into a reinsurance trust to collateralize and fund the obligations of Oaktown Re to NMIC under the reinsurance agreement. Funds in the reinsurance trust account are required to be invested in high credit quality money market funds at all times. We refer collectively to NMIC's reinsurance agreement with Oaktown Re and the issuance of the 2017 Notes by Oaktown Re as the 2017 ILN Transaction. Under the terms of the 2017 ILN Transaction, NMIC makes risk premium payments for the applicable outstanding reinsurance coverage amount and pays Oaktown Re for anticipated operating expenses (capped at $300 thousand per year). For the three and nine months ended September 30, 2018 , NMIC ceded risk premiums of $1.5 million and $4.8 million , respectively. For the three and nine months ended September 30, 2017 , NMIC ceded risk premiums of $1.9 million and $3.3 million , respectively. NMIC did not cede any losses to Oaktown Re during the three and nine month periods ended September 30, 2017 and 2018. Under the reinsurance agreement, NMIC holds an optional termination right if certain events occur, including, among others, a clean-up call if the outstanding reinsurance coverage amount amortizes to 10% or less of the reinsurance coverage amount at inception or if NMIC reasonably determines that changes to GSE or rating agency asset requirements would cause a material and adverse effect on the capital treatment afforded to NMIC under the agreement. In addition, there are certain events that will result in mandatory termination of the agreement, including NMIC's failure to pay premiums or consent to reductions in the trust account to make principal payments to noteholders, among others. 2018 ILN Transaction In July 2018, NMIC entered into a reinsurance agreement with Oaktown Re II that provides for up to $264.5 million of aggregate excess-of-loss reinsurance coverage at inception for new delinquencies on an existing portfolio of mortgage insurance policies written between January 1, 2017 and May 31, 2018. For the reinsurance coverage period, NMIC retains the first layer of $125.3 million of aggregate losses, of which all remained at September 30, 2018, and Oaktown Re II then provides second layer coverage up to the outstanding reinsurance coverage amount. NMIC retains losses in excess of the outstanding reinsurance coverage amount. The outstanding reinsurance coverage amount decreases from $264.5 million at inception over a ten-year period as the underlying covered mortgages are amortized or repaid, and/or the mortgage insurance coverage is canceled, and was $264.5 million as of September 30, 2018. The outstanding reinsurance coverage amount will begin amortizing after an initial period in which a target level of credit enhancement is obtained and will stop amortizing if certain credit enhancement or delinquency thresholds are triggered. Oaktown Re II financed the coverage by issuing mortgage insurance-linked notes in an aggregate amount of $264.5 million to unaffiliated investors (the 2018 Notes). The 2018 Notes mature on July 25, 2028. All of the proceeds paid to Oaktown Re II from the sale of the 2018 Notes were deposited into a reinsurance trust to collateralize and fund the obligations of Oaktown Re II to NMIC under the reinsurance agreement. Funds in the reinsurance trust account are required to be invested in high credit quality money market funds at all times. We refer collectively to NMIC's reinsurance agreement with Oaktown Re II and the issuance of the 2018 Notes by Oaktown Re II as the 2018 ILN Transaction, and the 2017 ILN Transaction and 2018 ILN Transaction as the ILN Transactions. Under the terms of the 2018 ILN Transaction, NMIC makes risk premium payments for the applicable outstanding reinsurance coverage amount and pays Oaktown Re II for anticipated operating expenses (capped at $250 thousand per year). For the three and nine months ended September 30, 2018, NMIC ceded risk premiums of $1.6 million . NMIC did not cede any losses to Oaktown Re II. Under the reinsurance agreement, NMIC holds an optional termination right if certain events occur, including, among others, a clean-up call if the outstanding reinsurance coverage amount amortizes to 10% or less of the reinsurance coverage amount at inception or if NMIC reasonably determines that changes to GSE or rating agency asset requirements would cause a material and adverse effect on the capital treatment afforded to NMIC under the agreement. In addition, there are certain events that will result in mandatory termination of the agreement, including NMIC's failure to pay premiums or consent to reductions in the trust account to make principal payments to noteholders, among others. Under the terms of the 2018 ILN Transaction, we are required to maintain a certain level of restricted funds in a premium deposit account with Bank of New York Mellon until the 2018 Notes have been redeemed in full. "Cash and cash equivalents" on our balance sheet includes restricted cash of $1.4 million as of September 30, 2018. We are not required to deposit additional funds into the premium deposit account and the restricted balance will decrease over time as the principal balance of the 2018 Notes declines. Quota share reinsurance 2016 QSR Transaction Effective September 1, 2016, NMIC entered into the 2016 QSR Transaction with a panel of third-party reinsurers. Each of the third-party reinsurers has an insurer financial strength rating of A- or better by Standard and Poor’s Rating Services (S&P), A.M. Best or both. Under the 2016 QSR Transaction, NMIC ceded premiums written related to: • 25% of existing risk written on eligible policies as of August 31, 2016; • 100% of existing risk under our pool agreement with Fannie Mae; and • 25% of risk on eligible policies written from September 1, 2016 through December 31, 2017. The 2016 QSR Transaction is scheduled to terminate on December 31, 2027, except with respect to the ceded pool risk, which is scheduled to terminate on August 31, 2023. However, NMIC has the option, based on certain conditions and subject to a termination fee, to terminate the agreement as of December 31, 2020, or at the end of any calendar quarter thereafter, which would result in NMIC reassuming the related risk. 2018 QSR Transaction Effective January 1, 2018, NMIC entered into the 2018 QSR Transaction with a panel of third-party reinsurers. Each of the third-party reinsurers has an insurer financial strength rating of A- or better by S&P, A.M. Best or both. Under the 2018 QSR Transaction, NMIC cedes premiums earned related to 25% of risk on eligible policies written in 2018 and 20% to 30% (such amount to be determined by NMIC at its sole election by December 1, 2018) in 2019. The 2018 QSR Transaction is scheduled to terminate on December 31, 2029. However, NMIC has the option, based on certain conditions and subject to a termination fee, to terminate the agreement as of December 31, 2022, or at the end of any calendar quarter thereafter, which would result in NMIC reassuming the related risk. The following table shows the amounts related to the QSR Transactions: For the three months ended For the nine months ended September 30, 2018 September 30, 2017 September 30, 2018 September 30, 2017 (In Thousands) Ceded risk-in-force 3,960,461 2,682,982 3,960,461 2,682,982 Ceded premiums written (16,546 ) (14,389 ) (46,389 ) (36,715 ) Ceded premiums earned (19,286 ) (13,393 ) (53,581 ) (34,721 ) Ceded claims and claims expenses 337 277 1,053 887 Ceding commission written 3,320 2,878 9,289 7,343 Ceding commission earned 3,814 2,581 10,501 6,921 Profit commission 11,272 7,758 31,180 19,945 Ceded premiums written under the 2016 QSR Transaction are recorded on the balance sheet as prepaid reinsurance premiums and amortized to ceded premiums earned in a manner consistent with the recognition of revenue on direct premiums. Under the 2018 QSR Transaction, premiums are ceded on an earned basis as defined in the agreement. NMIC receives a 20% ceding commission for premiums ceded under the QSR Transactions. NMIC also receives a profit commission, provided that the loss ratio on the loans covered under the 2016 QSR Transaction and 2018 QSR Transaction generally remains below 60% and 61% , respectively, as measured annually. Ceded claims and claim expenses under the QSR Transactions reduce NMIC's profit commission on a dollar-for-dollar basis. In accordance with the terms of the 2016 QSR Transaction, rather than making a cash payment or transferring investments for ceded premiums written, NMIC established a funds withheld liability, which also includes amounts due to NMIC for ceding and profit commissions. Any loss recoveries and any potential profit commission to NMIC will be realized from this account until exhausted. NMIC's reinsurance recoverable balance is further supported by trust accounts established and maintained by each reinsurer in accordance with the PMIERs funding requirements for risk ceded to non-affiliates. The reinsurance recoverable on loss reserves related to our 2016 QSR Transaction was $2.5 million as of September 30, 2018 . In accordance with the terms of the 2018 QSR Transaction, cash payments for ceded premiums earned are settled on a quarterly basis, offset by amounts due to NMIC for ceding and profit commissions. Any loss recoveries and any potential profit commission to NMIC are also settled quarterly. NMIC's reinsurance recoverable balance is supported by trust accounts established and maintained by each reinsurer in accordance with the PMIERs funding requirements for risk ceded to non-affiliates. The reinsurance recoverable on loss reserves related to our 2018 QSR Transaction was $64 thousand as of September 30, 2018 .

Reserves for Insurance Claims a

Reserves for Insurance Claims and Claim Expenses	9 Months Ended
	Sep. 30, 2018
Insurance [Abstract]
Reserves for Insurance Claims and Claim Expenses	Reserves for Insurance Claims and Claim Expenses We establish reserves to recognize the estimated liability for insurance claims and claim expenses related to defaults on insured mortgage loans. Consistent with industry practice, we establish reserves for loans that have been reported to us by servicers as having been in default for at least 60 days, referred to as case reserves, and additional loans that we estimate (based on actuarial review) have been in default for at least 60 days that have not yet been reported to us by servicers, referred to as incurred but not reported (IBNR) reserves. We also establish claims expense reserves, which represent the estimated cost of the claim administration process, including legal and other fees, as well as other general expenses of administering the claims settlement process. As of September 30, 2018 , we had reserves for insurance claims and claims expenses of $10.9 million for 746 primary loans in default. During the first nine months of 2018 , we paid 59 claims totaling $2.2 million , including 42 claims covered under the QSR Transactions representing $438 thousand of ceded claims and claims expenses. In 2013, we entered into a pool insurance transaction with Fannie Mae. The pool transaction includes a deductible, which represents the amount of claims to be absorbed by Fannie Mae before we are obligated to pay any claims. We only establish reserves for pool risk if we expect claims to exceed this deductible. At September 30, 2018 , 53 loans in the pool were past due by 60 days or more. These 53 loans represent approximately $3.2 million of risk-in-force (RIF). Due to the size of the remaining deductible, the low level of notices of default (NODs) reported on loans in the pool through September 30, 2018 and the expected severity (all loans in the pool have loan-to-value ratios (LTV) ratios under 80% ), we did not have any case or IBNR reserves for pool risks at September 30, 2018 . In connection with the settlement of pool claims, we applied $0.6 million to the pool deductible through September 30, 2018 . At September 30, 2018 , the remaining pool deductible was $9.8 million . We have not paid any pool claims to date. 100% of our pool RIF is reinsured under the 2016 QSR Transaction. The following table provides a reconciliation of the beginning and ending reserve balances for primary insurance claims and claim expenses: For the nine months ended September 30, 2018 2017 (In Thousands) Beginning balance $ 8,761 $ 3,001 Less reinsurance recoverables (1) (1,902 ) (297 ) Beginning balance, net of reinsurance recoverables 6,859 2,704 Add claims incurred: Claims and claim expenses incurred: Current year (2) 5,090 3,546 Prior years (3) (1,779 ) (581 ) Total claims and claims expenses incurred 3,311 2,965 Less claims paid: Claims and claim expenses paid: Current year (2) 37 — Prior years (3) 1,742 720 Total claims and claim expenses paid 1,779 720 Reserve at end of period, net of reinsurance recoverables 8,391 4,949 Add reinsurance recoverables (1) 2,517 1,174 Ending balance $ 10,908 $ 6,123 (1) Related to ceded losses recoverable on the QSR Transactions, included in "Other Assets" on the Condensed Consolidated Balance Sheets. See Note 5, "Reinsurance" for additional information. (2) Related to insured loans with their most recent defaults occurring in the current year. For example, if a loan had defaulted in a prior year and subsequently cured and later re-defaulted in the current year, that default would be included in the current year. (3) Related to insured loans with defaults occurring in prior years, which have been continuously in default since that time. The "claims incurred" section of the table above shows claims and claim expenses incurred on NODs for current and prior years, including IBNR reserves. The amount of claims incurred relating to current year NODs represents the estimated amount of claims and claims expenses to be ultimately paid on such loans in default. We recognized $1.8 million and $0.6 million of favorable prior year development during the nine months ended September 30, 2018 and 2017 , respectively, due to NOD cures and ongoing analysis of recent loss development trends. We may increase or decrease our original estimates as we learn additional information about individual defaults and claims and continue to observe and analyze loss development trends in our portfolio. Gross reserves of $3.7 million related to prior year defaults remained as of September 30, 2018 .

Earnings per Share (EPS)

Earnings per Share (EPS)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Earnings per Share (EPS)	Earnings per Share (EPS) Basic earnings per share is based on the weighted average number of shares of common stock outstanding, while diluted earnings per share is based on the weighted average number of shares of common stock outstanding and common stock equivalents that would be issuable upon the vesting of service based RSUs, and exercise of vested and unvested stock options and outstanding warrants. The following table reconciles the net income and the weighted average shares of common stock outstanding used in the computations of basic and diluted earnings per share of common stock: For the three months ended September 30, For the nine months ended September 30, 2018 2017 2018 2017 (In Thousands, except for per share data) Net income $ 24,811 $ 12,312 $ 72,407 $ 23,816 Basic weighted average shares outstanding 65,948 59,884 64,584 59,680 Basic earnings per share $ 0.38 $ 0.21 $ 1.12 $ 0.40 Net income $ 24,811 $ 12,312 $ 72,407 $ 23,816 Warrant gain, net of tax — — — — Diluted net income $ 24,811 $ 12,312 $ 72,407 $ 23,816 Basic weighted average shares outstanding 65,948 59,884 64,584 59,680 Dilutive effect of issuable shares 2,896 3,205 2,928 3,093 Diluted weighted average shares outstanding 68,844 63,089 67,512 62,773 Diluted earnings per share $ 0.36 $ 0.20 $ 1.07 $ 0.38 Anti-dilutive shares — 830 252 832

Warrants

Warrants	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Warrants	Warrants We issued 992 thousand warrants in connection with the Private Placement. Each warrant gives the holder thereof the right to purchase one share of common stock at an exercise price equal to $10.00 . The warrants were issued with an aggregate fair value of $5.1 million . During the nine months ended September 30, 2018 , 142 thousand warrants were exercised resulting in 87 thousand common shares issued. No warrants were exercised during the nine months ended September 30, 2017 . Upon exercise, we reclassified the fair value of the warrants from warrant liability to additional paid-in capital and recognized a loss of approximately $333 thousand . We account for these warrants to purchase our common shares in accordance with ASC 470-20, Debt with Conversion and Other Options and ASC 815-40, Derivatives and Hedging - Contracts in Entity's Own Equity.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes We are a U.S. taxpayer and are subject to a statutory U.S. federal corporate income tax rate of 21% for the current and all future years following the enactment of the TCJA on December 22, 2017. We were subject to a statutory U.S. federal corporate income tax rate of 35% for all prior years through December 31, 2017. NMIH files a consolidated U.S. federal and various state income tax returns on behalf of itself and its subsidiaries. Our provision for income taxes for interim reporting periods is established based on our estimated annual effective tax rates for a given year. Our effective tax rate on our pre-tax income was 22.1% and 20.2% for the three and nine months ended September 30, 2018 , respectively, compared to 36.9% and 33.3% for the three and nine months ended September 30, 2017 , respectively. The decrease in the effective tax rates for the three and nine months ended September 30, 2018 compared to the three and nine months ended September 30, 2017 is attributable to the decrease in the statutory U.S. federal corporate income tax rate. We currently pay no federal income tax primarily due to the forecasted utilization of our federal net operating loss carryforwards, which were $93.9 million as of December 31, 2017. As a result, the interim provision for income taxes represents changes to deferred tax assets. Provisional amounts The TCJA reduced the statutory U.S. federal corporate income tax rate from 35% to 21% and changed the tax deductibility of certain expenses for tax years beginning after December 31, 2017. We have not completed our full assessment of the tax effects of the enactment of the TCJA on our deferred tax balances as of September 30, 2018 and December 31, 2017; however, in certain cases, as described below, we have made reasonable estimates of the effects on our deferred tax balances. We recognized a $13.6 million income tax expense in the year ended December 31, 2017 for the items we could reasonably estimate. We are still analyzing the TCJA and refining our calculations, which could impact the measurement of our existing deferred tax assets including those related to share-based compensation. For tax years beginning after December 31, 2017, the TCJA expanded the number of individuals whose compensation is subject to a $1 million cap on tax deductibility and includes performance-based compensation in the calculation. As a result, we recorded a provisional amount to reduce the future tax benefit related to share-based compensation. We will continue to make and refine our calculations as additional analysis is completed. In addition, our estimates may also be affected as we incorporate additional guidance that may be issued by the U.S. Treasury Department, the IRS, or other standard-setting bodies. In February 2018, the FASB issued ASU 2018-02, Reclassification of Ce rtain Tax Effects from Accumulated Other Comprehensive Income (Topic 220). This update permits a company to reclassify the disproportionate income tax effects as a result of the TCJA on items within AOCI to retained earnings. We adopted this update on January 1, 2018 and adjusted the disproportionate incom e tax effects, or "stranded tax effects," resulting in a $0.3 million reduction to our beginning retained earnings as of January 1, 2018. The disproportionate tax effects that remain in AOCI of $4.2 million were not related to the TCJA and will remain in AOCI until certain events occur. Our elected accounting policy for available-for-sale debt securities is the "aggregate portfolio" approach.

Common Stock Offerings

Common Stock Offerings	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Common Stock Offerings	Common Stock Offerings In March 2018, we completed the sale of 3.7 million shares of common stock and granted the underwriters on the transaction a 15% overallotment option to purchase additional shares. The overallotment option was exercised in full, resulting in a total of 4.3 million shares of common stock issued. The common stock offering generated total proceeds of approximately $79.2 million , net of underwriting discounts, commissions and other direct offering expenses.

Regulatory Information

Regulatory Information	9 Months Ended
	Sep. 30, 2018
Insurance [Abstract]
Regulatory Information	Regulatory Information Our insurance subsidiaries, NMIC and Re One, file financial statements in conformity with statutory accounting principles (SAP) prescribed or permitted by the Wisconsin OCI, NMIC's principal regulator. Prescribed SAP includes state laws, regulations and general administrative rules, as well as a variety of publications of the National Association of Insurance Commissioners. The Wisconsin OCI recognizes only statutory accounting practices prescribed or permitted by the state of Wisconsin for determining and reporting the financial condition and results of operations of an insurance company and for determining its solvency under Wisconsin insurance laws. NMIC and Re One's combined statutory net income (loss) was as follows: For the three months ended September 30, For the nine months ended September 30, 2018 2017 2018 2017 (In Thousands) Statutory net (loss) $ (17,274 ) $ (7,688 ) $ (22,022 ) $ (29,394 ) NMIC and Re One's statutory surplus, contingency reserve and risk-to-capital (RTC) ratios were as follows: September 30, 2018 December 31, 2017 (Dollars In Thousands) Statutory surplus $ 440,697 $ 371,084 Contingency reserve 292,030 186,641 RTC Ratio 13.3:1 13.2:1 NMIH is not subject to any limitations on its ability to pay dividends except those generally applicable to corporations that are incorporated in Delaware. Delaware corporate law provides that dividends are only payable out of a corporation's surplus or, subject to certain limitations, recent net profits. NMIC and Re One's ability to pay dividends to NMIH is subject to Wisconsin OCI notice or approval. Certain other states in which NMIC is licensed also have statutes or regulations that restrict its ability to pay dividends. Since inception, NMIC and Re One have not paid any dividends to NMIH.

Organization, Basis of Presen_2

Organization, Basis of Presentation and Summary of Accounting Principles (Policies)	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements, which include the results of NMIH and its wholly owned subsidiaries, have been prepared in accordance with the instructions to Form 10-Q as prescribed by the SEC for interim reporting and include other information and disclosures required by accounting principles generally accepted in the U.S. (GAAP). Our accounts are maintained in U.S. dollars. These statements should be read in conjunction with our consolidated financial statements and notes thereto for the year ended December 31, 2017 , included in our 2017 10-K. All intercompany transactions have been eliminated. The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect reported amounts of assets and liabilities, as well as disclosure of contingent assets and liabilities as of the balance sheet date. Estimates also affect the reported amounts of income and expenses for the reporting period. Actual results could differ from those estimates. Certain reclassifications to our previously reported financial information have been made to conform to current period presentation. The results of operations for the interim period may not be indicative of the results that may be expected for the full year ending December 31, 2018 .
Reinsurance	Reinsurance We account for premiums, claims and claim expenses that are ceded to reinsurers on a basis consistent with those we use to account for the original policies we issue and pursuant to the terms of our reinsurance contracts. We account for premiums ceded or otherwise paid to reinsurers as reductions to premium revenue. Effective January 1, 2018, NMIC entered into a second quota share reinsurance transaction (2018 QSR Transaction) which is similar in nature to the quota share reinsurance transaction we entered into in September 2016 (2016 QSR Transaction, together with 2018 QSR Transaction, the QSR Transactions) (see Note 5, " Reinsurance "). We earn profit and ceding commissions in connection with the QSR Transactions. Profit commissions represent a percentage of the profits recognized by reinsurers that are returned to us, based on the level of claims and claim expenses that we cede. We recognize any profit commissions we earn as increases to premium revenue. Ceding commissions are calculated as a percentage of ceded written premiums under the 2016 QSR Transaction and as a percentage of ceded earned premiums under the 2018 QSR Transaction, to cover our costs to acquire and service the direct policies. We earn the ceding commissions in a manner consistent with our recognition of earnings on the underlying insurance policies, over the terms of the policies reinsured. We account for ceding commissions earned as a reduction to underwriting and operating expenses. Under the QSR Transactions, we cede a portion of claims and claim expenses reserves to our reinsurers, which are accounted for as reinsurance recoverables in "Other Assets" on the consolidated balance sheets and as reductions to claim expense on the consolidated statements of operations. We remain directly liable for all loss payments in the event we are unable to collect from any reinsurer.
Recent Accounting Pronouncements	Recent Accounting Pronouncements - Adopted In May 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2014-09, Revenue from Contracts with Customers (Topic 606). This update is intended to provide a consistent approach in recognizing revenue. In December 2016, the FASB clarified that all contracts that are within the scope of Topic 944, Financial Services-Insurance , are excluded from the scope of ASU 2014-09. Accordingly, this update did not impact the recognition of revenue related to insurance premiums or investment income, which represent a majority of our total revenues. The update impacted our loan review services revenue, which is the only revenue stream in scope of the update. We adopted this update on January 1, 2018 using the modified-retrospective approach and the impact was immaterial to our consolidated financial statements. In January 2016, the FASB issued ASU 2016-01, Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Financial Liabilities (Subtopic 825-10). This update requires entities to reduce the carrying amount of deferred tax assets, if necessary, by the amount of any tax benefit that is not expected to be realized. We adopted this update effective January 1, 2018. The impact was immaterial to our consolidated financial statements. In February 2018, the FASB issued ASU 2018-02, Reclassification of Ce rtain Tax Effects from Accumulated Other Comprehensive Income (Topic 220). This update permits a company to reclassify the disproportionate income tax effects as a result of the 2017 Tax Cuts and Jobs Act (the TCJA) on items within accumulated other comprehensive income (AOCI) to retained earnings. This standard will take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted. We adopted this update on January 1, 2018 and adjusted the disproportionate incom e tax effects, or "stranded tax effects," resulting in a $0.3 million reduction to our beginning retained earnings as of January 1, 2018. Recent Accounting Pronouncements - Not Yet Adopted In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). This update requires that businesses recognize rights and obligations associated with certain leases as assets and liabilities on the balance sheet. The standard also requires additional disclosures regarding the amount, timing, and uncertainty of cash flows arising from leases. For public business entities, this update is effective for annual periods beginning after December 15, 2018 and interim periods therein. Early adoption is permitted in any period. We expect to adopt this guidance on January 1, 2019. In September 2017, ASU 2017-13 added guidance from a SEC Staff Announcement, "Transition Related to Accounting Standards Update No. 2016-02," and in July 2018, the FASB issued ASU 2018-11, Leases (Topic 842), Targeted Improvement, which provides companies the option to apply the provisions of the new lease standard prospectively as of the effective date, without adjusting comparative periods presented. The effect of adoption will depend on our current lease portfolio at time of adoption; however, upon adoption, we anticipate that our reported assets and liabilities will increase in connection with the recognition of any right-of-use assets and lease liabilities, such as the operating lease on our corporate headquarters. In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326). This update requires companies to measure all expected credit losses for financial assets held at the reporting date. The standard also amends the accounting for credit losses on available-for-sale debt securities and purchased financial assets with credit deterioration. The standard will take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. We are currently reviewing the impacts the adoption of this guidance will have, if any, on our accounting for credit losses on our investment portfolio. We do not expect it to impact our accounting for insurance claims and claims expenses as these items are not in the scope of this ASU. In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), and Derivatives and Hedging (Topic 815). This update is intended to simplify the accounting for certain equity-linked financial instruments. The standard will take effect for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted. The guidance must be applied using a full or modified retrospective approach. We expect to adopt this ASU in the first quarter of 2019 and have determined that it will have no impact on our consolidated financial statements.
Earnings Per Share	Basic earnings per share is based on the weighted average number of shares of common stock outstanding, while diluted earnings per share is based on the weighted average number of shares of common stock outstanding and common stock equivalents that would be issuable upon the vesting of service based RSUs, and exercise of vested and unvested stock options and outstanding warrants.

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Schedule of Fair Values and Gross Unrealized Gains and Losses	Fair Values and Gross Unrealized Gains and Losses on Investments Amortized Gross Unrealized Fair Gains Losses As of September 30, 2018 (In Thousands) U.S. Treasury securities and obligations of U.S. government agencies $ 48,292 $ — $ (2,483 ) $ 45,809 Municipal debt securities 93,945 20 (1,933 ) 92,032 Corporate debt securities 536,240 323 (10,217 ) 526,346 Asset-backed securities 169,061 113 (1,236 ) 167,938 Total bonds 847,538 456 (15,869 ) 832,125 Short-term investments 42,256 54 — 42,310 Total investments $ 889,794 $ 510 $ (15,869 ) $ 874,435 Amortized Gross Unrealized Fair Gains Losses As of December 31, 2017 (In Thousands) U.S. Treasury securities and obligations of U.S. government agencies $ 65,669 $ — $ (981 ) $ 64,688 Municipal debt securities 89,973 534 (659 ) 89,848 Corporate debt securities 435,562 4,231 (1,958 ) 437,835 Asset-backed securities 100,153 916 (125 ) 100,944 Total bonds 691,357 5,681 (3,723 ) 693,315 Long-term investments - other 353 — — 353 Short-term investments 22,149 58 — 22,207 Total investments $ 713,859 $ 5,739 $ (3,723 ) $ 715,875
Schedule of Investments by Maturity	The amortized cost and fair values of available-for-sale securities as of September 30, 2018 and December 31, 2017 , by contractual maturity, are shown below. Expected maturities will differ from contractual maturities because issuers may have the right to call or prepay obligations with or without call or prepayment penalties. Because most asset-backed securities provide for periodic payments throughout their lives, they are listed below in a separate category. As of September 30, 2018 Amortized Fair (In Thousands) Due in one year or less $ 63,314 $ 63,368 Due after one through five years 335,262 330,603 Due after five through ten years 318,357 308,759 Due after ten years 3,800 3,767 Asset-backed securities 169,061 167,938 Total investments $ 889,794 $ 874,435 As of December 31, 2017 Amortized Fair (In Thousands) Due in one year or less $ 97,406 $ 97,394 Due after one through five years 195,795 195,626 Due after five through ten years 305,798 306,930 Due after ten years 14,707 14,981 Asset-backed securities 100,153 100,944 Total investments $ 713,859 $ 715,875
Schedule of Aging Unrealized Losses	For those securities in an unrealized loss position, the length of time the securities were in such a position is as follows: Less Than 12 Months 12 Months or Greater Total # of Securities Fair Value Unrealized Losses # of Securities Fair Value Unrealized Losses # of Securities Fair Value Unrealized Losses As of September 30, 2018 (Dollars in Thousands) U.S. Treasury securities and obligations of U.S. government agencies 5 $ 5,033 $ (60 ) 18 $ 40,776 $ (2,423 ) 23 $ 45,809 $ (2,483 ) Municipal debt securities 26 53,431 (885 ) 18 30,580 (1,048 ) 44 84,011 (1,933 ) Corporate debt securities 180 343,602 (6,123 ) 43 76,589 (4,094 ) 223 420,191 (10,217 ) Asset-backed securities 51 111,382 (976 ) 6 10,563 (260 ) 57 121,945 (1,236 ) Total 262 $ 513,448 $ (8,044 ) 85 $ 158,508 $ (7,825 ) 347 $ 671,956 $ (15,869 ) Less Than 12 Months 12 Months or Greater Total # of Securities Fair Value Unrealized Losses # of Securities Fair Value Unrealized Losses # of Securities Fair Value Unrealized Losses As of December 31, 2017 (Dollars in Thousands) U.S. Treasury securities and obligations of U.S. government agencies 16 $ 29,806 $ (394 ) 26 $ 34,882 $ (587 ) 42 $ 64,688 $ (981 ) Municipal debt securities 21 38,628 (264 ) 10 17,945 (395 ) 31 56,573 (659 ) Corporate debt securities 94 128,313 (829 ) 23 48,978 (1,129 ) 117 177,291 (1,958 ) Asset-backed securities 22 27,947 (63 ) 5 12,438 (62 ) 27 40,385 (125 ) Total 153 $ 224,694 $ (1,550 ) 64 $ 114,243 $ (2,173 ) 217 $ 338,937 $ (3,723 )
Schedule of Net Investment Income	The following table presents the components of net investment income: For the three months ended September 30, For the nine months ended September 30, 2018 2017 2018 2017 (In Thousands) Investment income $ 6,473 $ 4,363 $ 17,192 $ 12,455 Investment expenses (196 ) (193 ) (606 ) (570 ) Net investment income $ 6,277 $ 4,170 $ 16,586 $ 11,885
Schedule of Net Realized Investment Gains (Losses)	The following table presents the components of net realized investment gains (losses): For the three months ended September 30, For the nine months ended September 30, 2018 2017 2018 2017 (In Thousands) Gross realized investment gains $ 461 $ 69 $ 520 $ 536 Gross realized investment losses (469 ) — (469 ) (338 ) Net realized investment gains (losses) $ (8 ) $ 69 $ 51 $ 198

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Schedule of Fair Value Measurements of Financial Instruments	The following tables present the level within the fair value hierarchy at which our financial instruments were measured: Fair Value Measurements Using Quoted Prices in Significant Other Significant Fair Value As of September 30, 2018 (In Thousands) U.S. Treasury securities and obligations of U.S. government agencies $ 45,809 $ — $ — $ 45,809 Municipal debt securities — 92,032 — 92,032 Corporate debt securities — 526,346 — 526,346 Asset-backed securities — 167,938 — 167,938 Long-term investment – other — — — Cash, cash equivalents and short-term investments 60,497 60,497 Total assets $ 106,306 $ 786,316 $ — $ 892,622 Warrant liability — — 10,930 10,930 Total liabilities $ — $ — $ 10,930 $ 10,930 Fair Value Measurements Using Quoted Prices in Significant Other Significant Fair Value As of December 31, 2017 (In Thousands) U.S. Treasury securities and obligations of U.S. government agencies $ 59,844 $ 4,844 $ — $ 64,688 Municipal debt securities — 89,848 — 89,848 Corporate debt securities — 437,835 — 437,835 Asset-backed securities — 100,944 — 100,944 Long-term investment - other 353 — — 353 Cash, cash equivalents and short-term investments 41,403 — — 41,403 Total assets $ 101,600 $ 633,471 $ — $ 735,071 Warrant liability — — 7,472 7,472 Total liabilities $ — $ — $ 7,472 $ 7,472
Roll-Forward of Level 3 Liabilities Measured at Fair Value	The following is a roll-forward of Level 3 liabilities measured at fair value: For the nine months ended September 30, Warrant Liability 2018 2017 (In Thousands) Balance, January 1 $ 7,472 $ 3,367 Change in fair value of warrant liability included in earnings 4,935 679 Issuance of common stock on warrant exercise (1,477 ) — Balance, September 30 $ 10,930 $ 4,046
Schedule of Key Inputs and Assumptions in Option-Pricing Model	The following table outlines the key inputs and assumptions used to calculate the fair value of the warrant liability in the Black-Scholes option-pricing model as of the dates indicated. As of September 30, 2018 2017 Common stock price $ 22.65 $ 12.40 Risk free interest rate 2.86 - 2.90% 1.66 % Expected life 2.50 - 3.56 Years 3.25 years Expected volatility 39.9 - 41.5% 30.6 % Dividend yield 0% 0%

Debt Obligations (Tables)

Debt Obligations (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Schedule of Future Principal Payments	Future principal payments due under the 2018 Term Loan as of September 30, 2018 are as follows: As of September 30, 2018 Principal (In thousands) 2018 $ 375 2019 1,500 2020 1,500 2021 1,500 2022 1,500 2023 143,250 Total $ 149,625

Reinsurance (Tables)

Reinsurance (Tables)	9 Months Ended
	Sep. 30, 2018
Reinsurance Disclosures [Abstract]
Schedule of Effects of Reinsurance	The effect of our reinsurance agreements on premiums written and earned is as follows: For the three months ended For the nine months ended September 30, 2018 September 30, 2017 September 30, 2018 September 30, 2017 (In Thousands) Net premiums written Direct $ 73,748 $ 56,217 $ 210,452 $ 142,134 Ceded (1) (8,367 ) (8,501 ) (21,598 ) (20,029 ) Net premiums written $ 65,381 $ 47,716 $ 188,854 $ 122,105 Net premiums earned Direct $ 76,513 $ 52,024 $ 210,725 $ 133,696 Ceded (1) (11,106 ) (7,505 ) (28,789 ) (18,035 ) Net premiums earned $ 65,407 $ 44,519 $ 181,936 $ 115,661 (1) Net of profit commission The following table shows the amounts related to the QSR Transactions: For the three months ended For the nine months ended September 30, 2018 September 30, 2017 September 30, 2018 September 30, 2017 (In Thousands) Ceded risk-in-force 3,960,461 2,682,982 3,960,461 2,682,982 Ceded premiums written (16,546 ) (14,389 ) (46,389 ) (36,715 ) Ceded premiums earned (19,286 ) (13,393 ) (53,581 ) (34,721 ) Ceded claims and claims expenses 337 277 1,053 887 Ceding commission written 3,320 2,878 9,289 7,343 Ceding commission earned 3,814 2,581 10,501 6,921 Profit commission 11,272 7,758 31,180 19,945

Reserves for Insurance Claims_2

Reserves for Insurance Claims and Claim Expenses (Tables)	9 Months Ended
	Sep. 30, 2018
Insurance [Abstract]
Reconciliation of Liability for Insurance Claims and Claims Expenses	The following table provides a reconciliation of the beginning and ending reserve balances for primary insurance claims and claim expenses: For the nine months ended September 30, 2018 2017 (In Thousands) Beginning balance $ 8,761 $ 3,001 Less reinsurance recoverables (1) (1,902 ) (297 ) Beginning balance, net of reinsurance recoverables 6,859 2,704 Add claims incurred: Claims and claim expenses incurred: Current year (2) 5,090 3,546 Prior years (3) (1,779 ) (581 ) Total claims and claims expenses incurred 3,311 2,965 Less claims paid: Claims and claim expenses paid: Current year (2) 37 — Prior years (3) 1,742 720 Total claims and claim expenses paid 1,779 720 Reserve at end of period, net of reinsurance recoverables 8,391 4,949 Add reinsurance recoverables (1) 2,517 1,174 Ending balance $ 10,908 $ 6,123 (1) Related to ceded losses recoverable on the QSR Transactions, included in "Other Assets" on the Condensed Consolidated Balance Sheets. See Note 5, "Reinsurance" for additional information. (2) Related to insured loans with their most recent defaults occurring in the current year. For example, if a loan had defaulted in a prior year and subsequently cured and later re-defaulted in the current year, that default would be included in the current year. (3) Related to insured loans with defaults occurring in prior years, which have been continuously in default since that time.

Earnings per Share (EPS) (Table

Earnings per Share (EPS) (Tables)	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Schedule of Earnings per Share, Basic and Diluted	The following table reconciles the net income and the weighted average shares of common stock outstanding used in the computations of basic and diluted earnings per share of common stock: For the three months ended September 30, For the nine months ended September 30, 2018 2017 2018 2017 (In Thousands, except for per share data) Net income $ 24,811 $ 12,312 $ 72,407 $ 23,816 Basic weighted average shares outstanding 65,948 59,884 64,584 59,680 Basic earnings per share $ 0.38 $ 0.21 $ 1.12 $ 0.40 Net income $ 24,811 $ 12,312 $ 72,407 $ 23,816 Warrant gain, net of tax — — — — Diluted net income $ 24,811 $ 12,312 $ 72,407 $ 23,816 Basic weighted average shares outstanding 65,948 59,884 64,584 59,680 Dilutive effect of issuable shares 2,896 3,205 2,928 3,093 Diluted weighted average shares outstanding 68,844 63,089 67,512 62,773 Diluted earnings per share $ 0.36 $ 0.20 $ 1.07 $ 0.38 Anti-dilutive shares — 830 252 832

Regulatory Information (Tables)

Regulatory Information (Tables)	9 Months Ended
	Sep. 30, 2018
Insurance [Abstract]
Schedule of Statutory Net Income (Loss), Surplus, Contingency Reserve and Risk-to-Capital Ratio	NMIC and Re One's combined statutory net income (loss) was as follows: For the three months ended September 30, For the nine months ended September 30, 2018 2017 2018 2017 (In Thousands) Statutory net (loss) $ (17,274 ) $ (7,688 ) $ (22,022 ) $ (29,394 ) NMIC and Re One's statutory surplus, contingency reserve and risk-to-capital (RTC) ratios were as follows: September 30, 2018 December 31, 2017 (Dollars In Thousands) Statutory surplus $ 440,697 $ 371,084 Contingency reserve 292,030 186,641 RTC Ratio 13.3:1 13.2:1

Organization, Basis of Presen_3

Organization, Basis of Presentation and Summary of Accounting Principles - Narrative (Details) $ in Millions	1 Months Ended
	Mar. 31, 2018shares	Nov. 30, 2013shares	Apr. 30, 2012USD ($)shares	Sep. 30, 2018state
Business Acquisition [Line Items]
Common stock offered and sold (in shares) | shares	4,300,000
Number of states in which the entity operates | state				50
Private Placement
Business Acquisition [Line Items]
Proceeds from issuance of common stock, net of stock issuance costs | $			$ 510
IPO
Business Acquisition [Line Items]
Common stock offered and sold (in shares) | shares		2,400,000
Overallotment Option
Business Acquisition [Line Items]
Overallotment option included in sale of stock (percent)	15.00%
MAC Financial Holding Corporation and Subsidiaries
Business Acquisition [Line Items]
Cash, common stock and warrants issued for acquisition | $			8.5
Liabilities assumed in acquisition | $			$ 1.3
Common Stock - Class A | Private Placement
Business Acquisition [Line Items]
Common stock offered and sold (in shares) | shares			55,000,000

Organization, Basis of Presen_4

Organization, Basis of Presentation and Summary of Accounting Principles - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements [Line Items]
Decrease in cumulative effect of change in accounting principle	$ 0	$ (388)	$ (515)
Retained Earnings
New Accounting Pronouncements [Line Items]
Decrease in cumulative effect of change in accounting principle	282		$ (515)
ASU No. 2018-02 | Retained Earnings
New Accounting Pronouncements [Line Items]
Decrease in cumulative effect of change in accounting principle	$ 300

Investments - Fair Values and G

Investments - Fair Values and Gross Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 889,794	$ 713,859
Gross Unrealized Gains	510	5,739
Gross Unrealized Losses	(15,869)	(3,723)
Fair value of securities	874,435	715,875
U.S. Treasury securities and obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	48,292	65,669
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2,483)	(981)
Fair value of securities	45,809	64,688
Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	93,945	89,973
Gross Unrealized Gains	20	534
Gross Unrealized Losses	(1,933)	(659)
Fair value of securities	92,032	89,848
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	536,240	435,562
Gross Unrealized Gains	323	4,231
Gross Unrealized Losses	(10,217)	(1,958)
Fair value of securities	526,346	437,835
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	169,061	100,153
Gross Unrealized Gains	113	916
Gross Unrealized Losses	(1,236)	(125)
Fair value of securities	167,938	100,944
Total bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	847,538	691,357
Gross Unrealized Gains	456	5,681
Gross Unrealized Losses	(15,869)	(3,723)
Fair value of securities	832,125	693,315
Long-term investment – other
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		353
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair value of securities		353
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	42,256	22,149
Gross Unrealized Gains	54	58
Gross Unrealized Losses	0	0
Fair value of securities	$ 42,310	$ 22,207

Investments - Scheduled Maturit

Investments - Scheduled Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 63,314	$ 97,406
Due after one through five years	335,262	195,795
Due after five through ten years	318,357	305,798
Due after ten years	3,800	14,707
Asset-backed securities	169,061	100,153
Amortized Cost	889,794	713,859
Fair Value
Due in one year or less	63,368	97,394
Due after one through five years	330,603	195,626
Due after five through ten years	308,759	306,930
Due after ten years	3,767	14,981
Asset-backed securities	167,938	100,944
Fair Value	$ 874,435	$ 715,875

Investments - Narrative (Detail

Investments - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position, accumulated loss		$ 15,869	$ 3,723
Unrealized loss position, 12 months or greater		7,825	2,173
Other-than-temporary impairment	$ 100
U.S. Treasury securities and obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Cash and investments held with various state insurance departments		5,300	7,000
Unrealized loss position, accumulated loss		2,483	981
Unrealized loss position, 12 months or greater		$ 2,423	$ 587

Investments - Aging of Unrealiz

Investments - Aging of Unrealized Losses (Details) $ in Thousands	Sep. 30, 2018USD ($)security	Dec. 31, 2017USD ($)security
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	262	153
Fair value, less than 12 months	$ 513,448	$ 224,694
Unrealized losses, less than 12 months	$ (8,044)	$ (1,550)
Number of securities,12 months or greater | security	85	64
Fair value, 12 months or greater	$ 158,508	$ 114,243
Unrealized losses, 12 months or greater	$ (7,825)	$ (2,173)
Number of securities, total | security	347	217
Fair value	$ 671,956	$ 338,937
Unrealized Losses	$ (15,869)	$ (3,723)
U.S. Treasury securities and obligations of U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	5	16
Fair value, less than 12 months	$ 5,033	$ 29,806
Unrealized losses, less than 12 months	$ (60)	$ (394)
Number of securities,12 months or greater | security	18	26
Fair value, 12 months or greater	$ 40,776	$ 34,882
Unrealized losses, 12 months or greater	$ (2,423)	$ (587)
Number of securities, total | security	23	42
Fair value	$ 45,809	$ 64,688
Unrealized Losses	$ (2,483)	$ (981)
Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	26	21
Fair value, less than 12 months	$ 53,431	$ 38,628
Unrealized losses, less than 12 months	$ (885)	$ (264)
Number of securities,12 months or greater | security	18	10
Fair value, 12 months or greater	$ 30,580	$ 17,945
Unrealized losses, 12 months or greater	$ (1,048)	$ (395)
Number of securities, total | security	44	31
Fair value	$ 84,011	$ 56,573
Unrealized Losses	$ (1,933)	$ (659)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	180	94
Fair value, less than 12 months	$ 343,602	$ 128,313
Unrealized losses, less than 12 months	$ (6,123)	$ (829)
Number of securities,12 months or greater | security	43	23
Fair value, 12 months or greater	$ 76,589	$ 48,978
Unrealized losses, 12 months or greater	$ (4,094)	$ (1,129)
Number of securities, total | security	223	117
Fair value	$ 420,191	$ 177,291
Unrealized Losses	$ (10,217)	$ (1,958)
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Number of securities, less than 12 months | security	51	22
Fair value, less than 12 months	$ 111,382	$ 27,947
Unrealized losses, less than 12 months	$ (976)	$ (63)
Number of securities,12 months or greater | security	6	5
Fair value, 12 months or greater	$ 10,563	$ 12,438
Unrealized losses, 12 months or greater	$ (260)	$ (62)
Number of securities, total | security	57	27
Fair value	$ 121,945	$ 40,385
Unrealized Losses	$ (1,236)	$ (125)

Investments - Net Investment In

Investments - Net Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investment Income, Net [Abstract]
Investment income	$ 6,473	$ 4,363	$ 17,192	$ 12,455
Investment expenses	(196)	(193)	(606)	(570)
Net investment income	$ 6,277	$ 4,170	$ 16,586	$ 11,885

Investments - Net Realized Inve

Investments - Net Realized Investment Gains (Losses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Gross realized investment gains	$ 461	$ 69	$ 520	$ 536
Gross realized investment losses	(469)	0	(469)	(338)
Net realized investment gains (losses)	$ (8)	$ 69	$ 51	$ 198

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Assets and Liabilities at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	$ 874,435	$ 715,875
U.S. Treasury securities and obligations of U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	45,809	64,688
Municipal debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	92,032	89,848
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	526,346	437,835
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	167,938	100,944
Long-term investment – other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities		353
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, cash equivalents and short-term investments	60,497	41,403
Total assets	892,622	735,071
Warrant liability	10,930	7,472
Total liabilities	10,930	7,472
Recurring | U.S. Treasury securities and obligations of U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	45,809	64,688
Recurring | Municipal debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	92,032	89,848
Recurring | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	526,346	437,835
Recurring | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	167,938	100,944
Recurring | Long-term investment – other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	0	353
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, cash equivalents and short-term investments	60,497	41,403
Total assets	106,306	101,600
Warrant liability	0	0
Total liabilities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities and obligations of U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	45,809	59,844
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Long-term investment – other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	0	353
Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, cash equivalents and short-term investments		0
Total assets	786,316	633,471
Warrant liability	0	0
Total liabilities	0	0
Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities and obligations of U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	0	4,844
Recurring | Significant Other Observable Inputs (Level 2) | Municipal debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	92,032	89,848
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	526,346	437,835
Recurring | Significant Other Observable Inputs (Level 2) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	167,938	100,944
Recurring | Significant Other Observable Inputs (Level 2) | Long-term investment – other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	0	0
Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, cash equivalents and short-term investments		0
Total assets	0	0
Warrant liability	10,930	7,472
Total liabilities	10,930	7,472
Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities and obligations of U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Municipal debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Long-term investment – other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of securities		$ 0

Fair Value of Financial Instr_4

Fair Value of Financial Instruments - Rollforward of Level 3 Liabilities (Details) - Significant Unobservable Inputs (Level 3) - Warrant Liability - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 7,472	$ 3,367
Change in fair value of warrant liability included in earnings	4,935	679
Issuance of common stock on warrant exercise	(1,477)	0
Ending balance	$ 10,930	$ 4,046

Fair Value of Financial Instr_5

Fair Value of Financial Instruments - Valuation Assumptions for Warrant Liabilities (Details) - Significant Unobservable Inputs (Level 3) - Valuation Technique, Option Pricing Model	Sep. 30, 2018$ / shares	Sep. 30, 2017$ / shares
Common stock price
Key Inputs and Assumptions, Valuation Techniques
Warrant liability, measurement input	22.65	12.40
Risk free interest rate
Key Inputs and Assumptions, Valuation Techniques
Warrant liability, measurement input		0.0166
Risk free interest rate | Minimum
Key Inputs and Assumptions, Valuation Techniques
Warrant liability, measurement input	0.0286
Risk free interest rate | Maximum
Key Inputs and Assumptions, Valuation Techniques
Warrant liability, measurement input	0.0290
Expected life
Key Inputs and Assumptions, Valuation Techniques
Warrant liability, expected life (in years)		3 years 3 months
Expected life | Minimum
Key Inputs and Assumptions, Valuation Techniques
Warrant liability, expected life (in years)	2 years 6 months
Expected life | Maximum
Key Inputs and Assumptions, Valuation Techniques
Warrant liability, expected life (in years)	3 years 6 months 21 days
Expected volatility
Key Inputs and Assumptions, Valuation Techniques
Warrant liability, measurement input		0.306
Expected volatility | Minimum
Key Inputs and Assumptions, Valuation Techniques
Warrant liability, measurement input	0.399
Expected volatility | Maximum
Key Inputs and Assumptions, Valuation Techniques
Warrant liability, measurement input	0.415
Dividend yield
Key Inputs and Assumptions, Valuation Techniques
Warrant liability, measurement input	0	0

Debt Obligations - Narrative (D

Debt Obligations - Narrative (Details) - USD ($)	May 24, 2018	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Nov. 10, 2015
Debt Instrument [Line Items]
Interest expense			$ 2,972,000	$ 3,352,000	$ 11,951,000	$ 10,146,000
Term loan			149,625,000		149,625,000
Secured Debt | 2018 Term Loan
Debt Instrument [Line Items]
Debt instrument face amount	$ 150,000,000
Debt instrument term (years)	5 years
Interest rate during period (percent)		6.99%
Frequency of periodic payment	Quarterly
Periodic payment of principal	$ 375,000
Prepayment premium (percent)	1.00%
Prepayment penalty time period (months)	6 months
Interest expense					11,600,000
Debt issuance costs	$ 1,800,000
Original issue discount	$ 986,000
Unamortized issuance cost and original issue discount			2,600,000		2,600,000
Term loan			149,600,000		149,600,000
Debt instrument covenant, maximum debt-to-total capitalization ratio		35.00%
Secured Debt | 2018 Term Loan | Eurodollar
Debt Instrument [Line Items]
Variable rate floor (percent)		1.00%
Basis spread on variable rate (percent)		4.75%
Senior Secured Debt | 2015 Term Loan
Debt Instrument [Line Items]
Debt instrument face amount							$ 150,000,000
Costs related to extinguishment of debt					2,200,000
Revolving Credit Facility | 2018 Revolving Credit Facility
Debt Instrument [Line Items]
Debt instrument term (years)	3 years
Credit facility borrowing capacity	$ 85,000,000
Debt issuance costs	$ 1,500,000
Debt instrument covenant, maximum debt-to-total capitalization ratio		35.00%
Borrowings outstanding			0		0
Commitment fee (percent)		0.50%
Commitment fees in interest expense			100,000		200,000
Interest expense from amortization of debt issuance costs			100,000		200,000
Remaining net debt issuance costs			$ 1,300,000		$ 1,300,000
Revolving Credit Facility | 2018 Revolving Credit Facility | Minimum
Debt Instrument [Line Items]
Commitment fee (percent)	0.30%
Revolving Credit Facility | 2018 Revolving Credit Facility | Maximum
Debt Instrument [Line Items]
Commitment fee (percent)	0.60%
Revolving Credit Facility | 2018 Revolving Credit Facility | Base Rate
Debt Instrument [Line Items]
Variable rate floor (percent)	1.00%
Revolving Credit Facility | 2018 Revolving Credit Facility | Base Rate | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.00%
Revolving Credit Facility | 2018 Revolving Credit Facility | Base Rate | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	2.50%
Revolving Credit Facility | 2018 Revolving Credit Facility | Eurodollar
Debt Instrument [Line Items]
Variable rate floor (percent)	0.00%
Revolving Credit Facility | 2018 Revolving Credit Facility | Eurodollar | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	2.00%
Revolving Credit Facility | 2018 Revolving Credit Facility | Eurodollar | Maximum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	3.50%

Debt Obligations - Schedule of

Debt Obligations - Schedule of Future Principal Payments (Details) $ in Thousands	Sep. 30, 2018USD ($)
Future Principal Payments [Abstract]
2,018	$ 375
2,019	1,500
2,020	1,500
2,021	1,500
2,022	1,500
2,023	143,250
Total	$ 149,625

Reinsurance - Narrative (Detail

Reinsurance - Narrative (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jul. 31, 2018	May 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Restricted cash	$ 1,406			$ 1,406		$ 1,406		$ 0
Reinsurance recoverable on unpaid claims	$ 2,517			$ 2,517	$ 1,174	$ 2,517	$ 1,174	$ 1,902	$ 297
2017 ILN Transaction
Ceded Credit Risk [Line Items]
Optional termination right, percent of reinsurance coverage threshold	10.00%			10.00%		10.00%
Mortgage-linked Debt | 2017 ILN Notes | Oaktown Re Ltd
Ceded Credit Risk [Line Items]
Proceeds from issuance of notes			$ 211,300
Mortgage-linked Debt | 2018 ILN Notes | Oaktown Re Ltd
Ceded Credit Risk [Line Items]
Proceeds from issuance of notes		$ 264,500
Third-Party Reinsurers | 2017 ILN Transaction
Ceded Credit Risk [Line Items]
Aggregate excess of loss reinsurance coverage	$ 144,100
Aggregate excess of loss reinsurance retained by company	125,600		126,800
Risk premiums paid				$ 1,500	$ 1,900	$ 4,800	$ 3,300
Third-Party Reinsurers | 2017 ILN Transaction | Maximum
Ceded Credit Risk [Line Items]
Aggregate excess of loss reinsurance coverage			$ 211,300
Reinsurance coverage, term of underlying mortgage amortization (in years)			10 years
Anticipated payment related to annual operating expenses						300
Third-Party Reinsurers | 2018 ILN Transaction
Ceded Credit Risk [Line Items]
Aggregate excess of loss reinsurance coverage	264,500
Aggregate excess of loss reinsurance retained by company		125,300
Risk premiums paid				1,600		1,600
Third-Party Reinsurers | 2018 ILN Transaction | Maximum
Ceded Credit Risk [Line Items]
Aggregate excess of loss reinsurance coverage		$ 264,500
Reinsurance coverage, term of underlying mortgage amortization (in years)		10 years
Anticipated payment related to annual operating expenses						$ 250
Third-Party Reinsurers | QSR Transactions
Ceded Credit Risk [Line Items]
Ceding commissions under 2016 QSR Transaction						20.00%
Third-Party Reinsurers | 2018 QSR Transaction
Ceded Credit Risk [Line Items]
Threshold for loss ratio on loans to qualify for profit commission						61.00%
Reinsurance recoverable on unpaid claims	64			64		$ 64
Third-Party Reinsurers | Risk Written Policies in 2018
Ceded Credit Risk [Line Items]
Percent of premiums earned under 2018 QSR Transaction						25.00%
Third-Party Reinsurers | Risk Written Policies in 2019 | Maximum
Ceded Credit Risk [Line Items]
Percent of premiums earned under 2018 QSR Transaction						30.00%
Third-Party Reinsurers | Risk Written Policies in 2019 | Minimum
Ceded Credit Risk [Line Items]
Percent of premiums earned under 2018 QSR Transaction						20.00%
Third-Party Reinsurers | 2016 QSR Transaction
Ceded Credit Risk [Line Items]
Threshold for loss ratio on loans to qualify for profit commission						60.00%
Reinsurance recoverable on unpaid claims	$ 2,500			$ 2,500		$ 2,500
Third-Party Reinsurers | Existing Risk Written Policies
Ceded Credit Risk [Line Items]
Percent of premiums ceded under 2016 QSR Transaction						25.00%
Third-Party Reinsurers | Fannie Mae
Ceded Credit Risk [Line Items]
Percent of premiums ceded under 2016 QSR Transaction						100.00%
Third-Party Reinsurers | Risk Written Policies from September 1, 2016 through December 31, 2017
Ceded Credit Risk [Line Items]
Percent of premiums ceded under 2016 QSR Transaction						25.00%

Reinsurance - Effect of Reinsur

Reinsurance - Effect of Reinsurance on Net Premiums Written and Earned (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net premiums written
Direct	$ 73,748	$ 56,217	$ 210,452	$ 142,134
Ceded	(8,367)	(8,501)	(21,598)	(20,029)
Net premiums written	65,381	47,716	188,854	122,105
Net premiums earned
Direct	76,513	52,024	210,725	133,696
Ceded	(11,106)	(7,505)	(28,789)	(18,035)
Net premiums earned	$ 65,407	$ 44,519	$ 181,936	$ 115,661

Reinsurance - Amounts Ceded Rel

Reinsurance - Amounts Ceded Related to QSR Transactions (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Ceded Credit Risk [Line Items]
Ceded premiums written	$ (8,367)	$ (8,501)	$ (21,598)	$ (20,029)
Ceded premiums earned	(11,106)	(7,505)	(28,789)	(18,035)
Third-Party Reinsurers
Ceded Credit Risk [Line Items]
Ceded risk-in-force	3,960,461	2,682,982	3,960,461	2,682,982
Ceded premiums written	(16,546)	(14,389)	(46,389)	(36,715)
Ceded premiums earned	(19,286)	(13,393)	(53,581)	(34,721)
Ceded claims and claims expenses	337	277	1,053	887
Ceding commission written	3,320	2,878	9,289	7,343
Ceding commission earned	3,814	2,581	10,501	6,921
Profit commission	$ 11,272	$ 7,758	$ 31,180	$ 19,945

Reserves for Insurance Claims_3

Reserves for Insurance Claims and Claim Expenses - Narrative (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)loanclaim	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
IBNR, default period (at least)	60 days
Total gross liability for unpaid claims and claim adjustment expenses	$ 10,908	$ 6,123	$ 8,761	$ 3,001
Primary loans in default | loan	746
Number of claims paid | claim	59
Claims paid, including amounts covered by insurance	$ 2,200
Favorable prior year development	1,779	$ 581
Reserve for prior year insurance claims and claim expenses	$ 3,700
Fannie Mae
Liability for Claims and Claims Adjustment Expense [Line Items]
Number of loans in pool past due 60 days or more | loan	53
Risk-in-Force of loans in pool past due 60 days or more	$ 3,200
Loan-to-value ratio (less than)	0.8
Claims applied to pool deductible	$ 600
Deductible on policy	9,800
QSR Transactions
Liability for Claims and Claims Adjustment Expense [Line Items]
Component of claims paid covered under QSR Transaction	$ 438
2016 QSR Transaction
Liability for Claims and Claims Adjustment Expense [Line Items]
Number of covered claims included in number of claims paid | claim	42
2016 QSR Transaction | Fannie Mae
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of pool RIF reinsured	100.00%

Reserves for Insurance Claims_4

Reserves for Insurance Claims and Claim Expenses - Reconciliation of Reserve Balances for Insurance Claims Expense (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning balance	$ 8,761	$ 3,001
Less reinsurance recoverables	1,902	297
Beginning balance, net of reinsurance recoverables	6,859	2,704
Claims incurred:
Current year	5,090	3,546
Prior years	(1,779)	(581)
Total claims and claims expenses incurred	3,311	2,965
Claims and claim expenses paid:
Current year	37	0
Prior years	1,742	720
Total claims and claim expenses paid	1,779	720
Ending balance, net of reinsurance recoverables	8,391	4,949
Add reinsurance recoverables	2,517	1,174
Ending balance	$ 10,908	$ 6,123

Earnings per Share (EPS) - Comp

Earnings per Share (EPS) - Computation of Basic and Diluted Earnings (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings per Share, Basic [Abstract]
Net income	$ 24,811	$ 25,241	$ 22,355	$ 12,312	$ 6,012	$ 5,492	$ 72,407	$ 23,816
Basic weighted average shares outstanding (in shares)	65,948			59,884			64,584	59,680
Basic earnings per share (in dollars per share)	$ 0.38			$ 0.21			$ 1.12	$ 0.40
Earnings per Share, Diluted [Abstract]
Net income	$ 24,811	$ 25,241	$ 22,355	$ 12,312	$ 6,012	$ 5,492	$ 72,407	$ 23,816
Warrant gain, net of tax	0			0			0	0
Diluted net income	$ 24,811			$ 12,312			$ 72,407	$ 23,816
Basic weighted average shares outstanding (in shares)	65,948			59,884			64,584	59,680
Dilutive effect of issuable shares (in shares)	2,896			3,205			2,928	3,093
Dilutive weighted average shares outstanding (in shares)	68,844			63,089			67,512	62,773
Diluted earnings per share (in dollars per share)	$ 0.36			$ 0.20			$ 1.07	$ 0.38
Antidilutive securities excluded from EPS calculation (in shares)	0			830			252	832

Warrants - Narrative (Details)

Warrants - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Apr. 30, 2012
Debt Disclosure [Abstract]
Warrants issued (in shares)						992,000
Right to purchase, number of shares per warrant						1
Exercise price of warrants (in dollars per warrant)						$ 10
Warrant liability, at fair value			$ 10,930		$ 7,472	$ 5,100
Warrants exercised (in shares)			142,000	0
Stock issued upon exercise of warrants (in shares)	3,751	25,686	87,000
Loss of exercise of warrants			$ 333

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Effective income tax rate on pre-tax income or loss	22.10%	36.90%	20.20%	33.30%
Net operating loss carryforwards					$ 93,900
Provisional income tax expense related to Tax Cuts and Jobs Act of 2017					13,600
Disproportionate tax effects in AOCI	$ 4,200		$ 4,200
New Accounting Pronouncements [Line Items]
Cumulative effect of change in accounting principle					0	$ (388)	$ (515)
Retained Earnings
New Accounting Pronouncements [Line Items]
Cumulative effect of change in accounting principle					282		$ (515)
ASU No. 2018-02 | Retained Earnings
New Accounting Pronouncements [Line Items]
Cumulative effect of change in accounting principle					$ 300

Common Stock Offerings - Narrat

Common Stock Offerings - Narrative (Details) - USD ($) $ in Thousands, shares in Millions	1 Months Ended	9 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Class of Stock [Line Items]
Common stock sold excluding overallotment shares (in shares)	3.7
Common stock offered and sold (in shares)	4.3
Proceeds from issuance of common stock	$ 79,200	$ 79,165	$ 0
Overallotment Option
Class of Stock [Line Items]
Overallotment option included in sale of stock (percent)	15.00%

Regulatory Information - Statut

Regulatory Information - Statutory Net Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Combined
Statutory Accounting Practices [Line Items]
Statutory net (loss)	$ (17,274)	$ (7,688)	$ (22,022)	$ (29,394)

Regulatory Information - Schedu

Regulatory Information - Schedule of Statutory Surplus, Contingency Reserve and RTC Ratio (Details) - Combined $ in Thousands	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Statutory Accounting Practices [Line Items]
Statutory surplus	$ 440,697	$ 371,084
Contingency reserve	$ 292,030	$ 186,641
RTC Ratio	13.3	13.2