Summary of Significant Accounting Policies - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of acquisition costs for individual life contracts, first term	10 years
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,290,550	$ 1,501,573
Other comprehensive income (loss) before reclassifications	259,289	(200,090)	$ 71,978
Amounts reclassified from AOCI	(124,364)	12,927	3,161
Cumulative effect of change in accounting principle	0	(15,041)	0
Reclassification of deferred taxes			47,900
Net current period other comprehensive income (loss)	134,925	(202,204)	123,039
Ending balance	1,567,285	1,290,550	1,501,573
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	84,756	286,960	163,921
Cumulative effect of change in accounting principle	0	(15,041)	0
Reclassification of deferred taxes	0	0	47,900
Ending balance	219,681	84,756	286,960
Net unrealized investment gains (losses) on securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	96,941	300,177	175,738
Other comprehensive income (loss) before reclassifications	257,871	(201,122)	71,244
Amounts reclassified from AOCI	(124,364)	12,927	3,161
Cumulative effect of change in accounting principle	15,000	(15,041)
Reclassification of deferred taxes			50,034
Net current period other comprehensive income (loss)	133,507	(203,236)	124,439
Ending balance	230,448	96,941	300,177
Net Funded Status of Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(12,185)	(13,217)	(11,817)
Other comprehensive income (loss) before reclassifications	1,418	1,032	734
Amounts reclassified from AOCI	0	0	0
Cumulative effect of change in accounting principle		0
Reclassification of deferred taxes			(2,134)
Net current period other comprehensive income (loss)	1,418	1,032	(1,400)
Ending balance	$ (10,767)	$ (12,185)	$ (13,217)