Basis of Presentation - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,290,550
Other comprehensive income (loss) before reclassifications	120,303	$ (109,539)
Amounts reclassified from AOCI	(6,405)	2,443
Cumulative effect of change in accounting principle	0	(15,041)
Net current period other comprehensive income (loss)	113,898	(122,137)
Ending balance	1,423,714	1,404,679
Other income tax expense (benefit)	1,703	(649)
Reclassification of unrealized gains on equity securities, net of tax	0	15,041
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	84,756	286,960
Cumulative effect of change in accounting principle	0	(15,041)
Ending balance	198,654	164,823
Net Unrealized Investment Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	96,941	300,177
Other comprehensive income (loss) before reclassifications	120,303	(109,539)
Amounts reclassified from AOCI	(6,405)	2,443
Cumulative effect of change in accounting principle		(15,041)
Net current period other comprehensive income (loss)	113,898	(122,137)
Ending balance	210,839	178,040
Pretax reclassification amounts from accumulated other comprehensive income (loss)	8,108	(3,092)
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(12,185)	(13,217)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from AOCI	0	0
Cumulative effect of change in accounting principle		0
Net current period other comprehensive income (loss)	0	0
Ending balance	$ (12,185)	$ (13,217)