Fair Value of Financial Instruments, Fair Value Inputs (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | $ 2,367,400 | $ 2,485,900 |
Equity Securities, FV-NI | 205,700 | 256,700 |
Short-term Investments | 9,300 | 0 |
Investments | 2,620,500 | 2,778,400 |
Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 205,700 | 256,700 |
Short-term Investments | 9,300 | 0 |
Investments | 215,000 | 256,700 |
Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 0 | 0 |
Short-term Investments | | 0 |
Investments | 2,367,400 | 2,485,900 |
Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 0 | 0 |
Short-term Investments | 0 | 0 |
Investments | 0 | 0 |
Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 2,367,400 | 2,485,900 |
Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US Treasury Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 91,100 | 85,600 |
US Treasury Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US Treasury Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 91,100 | 85,600 |
US Treasury Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US Government Agencies Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 3,100 | 2,900 |
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 3,100 | 2,900 |
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US States and Political Subdivisions Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 422,400 | 484,500 |
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 422,400 | 484,500 |
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Corporate Debt Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 971,900 | 1,079,000 |
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 971,900 | 1,079,000 |
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Residential Mortgage Backed Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 497,600 | 480,400 |
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 497,600 | 480,400 |
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Commercial Mortgage Backed Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 105,000 | 110,600 |
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 105,000 | 110,600 |
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Asset-backed Securities [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 49,900 | 61,200 |
Asset-backed Securities [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Asset-backed Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 49,900 | 61,200 |
Asset-backed Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Collateralized Loan Obligations [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 74,300 | |
Collateralized Loan Obligations [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Collateralized Loan Obligations [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 74,300 | 0 |
Collateralized Loan Obligations [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Other Debt Obligations [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 152,100 | 181,700 |
Other Debt Obligations [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Other Debt Obligations [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 152,100 | 181,700 |
Other Debt Obligations [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturity securities, fair value | 0 | 0 |
Industrial and Miscellaneous [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 172,300 | 216,400 |
Industrial and Miscellaneous [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 172,300 | 216,400 |
Industrial and Miscellaneous [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 0 | 0 |
Industrial and Miscellaneous [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 0 | 0 |
Other [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 33,400 | 40,300 |
Other [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 33,400 | 40,300 |
Other [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | 0 | 0 |
Other [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity Securities, FV-NI | $ 0 | $ 0 |