Investments (Available-For-Sale Fixed Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Reclassifications from AOCI to net income:
Amortized Cost	$ 2,251,040	$ 2,116,825
Gross Unrealized Gains	54,314	9,679
Gross Unrealized Losses	3,819	32,706
Estimated Fair Value	2,301,535	2,093,798
U.S. Treasury obligations
Reclassifications from AOCI to net income:
Amortized Cost	118,938	121,274
Gross Unrealized Gains	2,095	331
Gross Unrealized Losses	113	1,404
Estimated Fair Value	120,920	120,201
U.S. Government-sponsored enterprise obligations
Reclassifications from AOCI to net income:
Amortized Cost	22,195	35,758
Gross Unrealized Gains	147	25
Gross Unrealized Losses	8	429
Estimated Fair Value	22,334	35,354
State and municipal bonds
Reclassifications from AOCI to net income:
Amortized Cost	282,398	289,544
Gross Unrealized Gains	11,093	4,877
Gross Unrealized Losses	94	649
Estimated Fair Value	293,397	293,772
Corporate debt
Reclassifications from AOCI to net income:
Amortized Cost	1,318,250	1,244,577
Gross Unrealized Gains	33,313	3,328
Gross Unrealized Losses	2,914	24,430
Estimated Fair Value	1,348,649	1,223,475
Residential mortgage-backed securities
Reclassifications from AOCI to net income:
Amortized Cost	199,386	184,463
Gross Unrealized Gains	2,980	814
Gross Unrealized Losses	523	4,039
Estimated Fair Value	201,843	181,238
Agency commercial mortgage-backed securities
Reclassifications from AOCI to net income:
Amortized Cost	9,926	13,296
Gross Unrealized Gains	274	12
Gross Unrealized Losses	18	200
Estimated Fair Value	10,182	13,108
Other commercial mortgage-backed securities
Reclassifications from AOCI to net income:
Amortized Cost	70,579	31,330
Gross Unrealized Gains	2,061	38
Gross Unrealized Losses	103	375
Estimated Fair Value	72,537	30,993
Other asset-backed securities
Reclassifications from AOCI to net income:
Amortized Cost	229,368	196,583
Gross Unrealized Gains	2,351	254
Gross Unrealized Losses	46	1,180
Estimated Fair Value	$ 231,673	$ 195,657