Investments - Available-For-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 2,241,304	$ 2,116,825
Gross Unrealized Gains	50,565	9,679
Gross Unrealized Losses	3,084	32,706
Fixed maturities, available for sale	2,288,785	2,093,798
U.S. Treasury obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	109,060	121,274
Gross Unrealized Gains	1,533	331
Gross Unrealized Losses	126	1,404
Fixed maturities, available for sale	110,467	120,201
U.S. Government-sponsored enterprise obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,215	35,758
Gross Unrealized Gains	125	25
Gross Unrealized Losses	0	429
Fixed maturities, available for sale	17,340	35,354
State and municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	287,658	289,544
Gross Unrealized Gains	9,110	4,877
Gross Unrealized Losses	675	649
Fixed maturities, available for sale	296,093	293,772
Corporate debt
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,308,889	1,244,577
Gross Unrealized Gains	33,050	3,328
Gross Unrealized Losses	1,575	24,430
Fixed maturities, available for sale	1,340,364	1,223,475
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	205,588	184,463
Gross Unrealized Gains	3,139	814
Gross Unrealized Losses	319	4,039
Fixed maturities, available for sale	208,408	181,238
Agency commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,054	13,296
Gross Unrealized Gains	182	12
Gross Unrealized Losses	15	200
Fixed maturities, available for sale	8,221	13,108
Other commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	70,621	31,330
Gross Unrealized Gains	1,468	38
Gross Unrealized Losses	221	375
Fixed maturities, available for sale	71,868	30,993
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	234,219	196,583
Gross Unrealized Gains	1,958	254
Gross Unrealized Losses	153	1,180
Fixed maturities, available for sale	$ 236,024	$ 195,657

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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