Deferred Acquisition Costs (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Movement Analysis of Deferred Policy Acquisition Costs
Balance at beginning of year				$ 4,137,925				$ 3,958,063	$ 4,137,925	$ 3,958,063	$ 3,783,158
Deferred during period:
Commissions									534,735	497,459	465,920
Other expenses									218,926	202,092	194,214
Total deferred									753,661	699,551	660,134
Foreign exchange adjustment									4,299	0	5,712
Adjustment attributable to unrealized investment losses									0	5,549	0
Total additions									757,960	705,100	665,846
Amortized during period	$ (139,290)	$ (138,449)	$ (138,165)	$ (135,822)	$ (128,501)	$ (129,492)	$ (129,077)	$ (129,620)	(551,726)	(516,690)	(490,403)
Foreign exchange adjustment									0	(8,548)	0
Adjustment attributable to unrealized investment gains									(2,218)	0	(538)
Total deductions									(553,944)	(525,238)	(490,941)
Balance at end of year	$ 4,341,941				$ 4,137,925				$ 4,341,941	$ 4,137,925	$ 3,958,063