Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,567,285
Other comprehensive income (loss) before reclassifications	(104,530)	$ 120,303
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	10,761	(6,405)
Net current period other comprehensive income (loss)	(93,769)	113,898
Ending balance	1,477,620	1,423,714
Other income tax expense (benefit)	(2,900)	1,700
Net Unrealized Investment Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	230,448	96,941
Other comprehensive income (loss) before reclassifications	(104,530)	120,303
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	10,761	(6,405)
Net current period other comprehensive income (loss)	(93,769)	113,898
Ending balance	136,679	210,839
Pretax reclassification amounts from accumulated other comprehensive income (loss)	(13,600)	8,100
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(10,767)	(12,185)
Other comprehensive income (loss) before reclassifications	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	0
Net current period other comprehensive income (loss)	0	0
Ending balance	(10,767)	(12,185)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	219,681	84,756
Ending balance	$ 125,912	$ 198,654