Basis of Presentation - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,290,550
Other comprehensive income (loss) before reclassifications	$ (64,577)	$ 49,165	(292,043)	$ 211,577
Amounts reclassified from AOCI	(1,273)	(473)	(122,603)	2,399
Net current period other comprehensive income (loss)	63,304	(49,638)	169,440	(224,219)
Cumulative effect of change in accounting principle	0	0	0	(15,041)
Ending balance	1,578,612	1,304,256	1,578,612	1,304,256
Other income tax expense (benefit)	339	126	32,591	(638)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	190,892	112,379	84,756	286,960
Cumulative effect of change in accounting principle	0	0	0	(15,041)
Ending balance	254,196	62,741	254,196	62,741
Net Unrealized Investment Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	203,077	125,596	96,941	300,177
Other comprehensive income (loss) before reclassifications	(64,577)	49,165	(292,043)	211,577
Amounts reclassified from AOCI	(1,273)	(473)	(122,603)	2,399
Net current period other comprehensive income (loss)	63,304	(49,638)	169,440	(224,219)
Cumulative effect of change in accounting principle				(15,041)
Ending balance	266,381	75,958	266,381	75,958
Pretax reclassification amounts from accumulated other comprehensive income (loss)	1,612	599	155,194	(3,037)
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(12,185)	(13,217)	(12,185)	(13,217)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI	0	0	0	0
Net current period other comprehensive income (loss)	0	0	0	0
Cumulative effect of change in accounting principle				0
Ending balance	$ (12,185)	$ (13,217)	$ (12,185)	$ (13,217)