Deferred Acquisition Costs (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement Analysis of Deferred Policy Acquisition Costs
Balance at beginning of year				$ 3,783,158				$ 3,617,135	$ 3,783,158	$ 3,617,135	$ 3,457,397
Commissions									465,920	436,252	401,166
Other expenses									194,214	199,066	211,015
Total deferred									660,134	635,318	612,181
Foreign exchange adjustment									5,712	2,180	0
Adjustment attributable to unrealized investment losses									0	0	8,682
Total additions									665,846	637,498	620,863
Amortized during period	$ (120,040)	$ (122,334)	$ (122,121)	$ (125,908)	$ (116,191)	$ (116,821)	$ (117,245)	$ (118,806)	(490,403)	(469,063)	(445,625)
Foreign exchange adjustment									0	0	(15,500)
Adjustment attributable to unrealized investment gains									(538)	(2,412)	0
Total deductions									(490,941)	(471,475)	(461,125)
Balance at end of year	$ 3,958,063				$ 3,783,158				$ 3,958,063	$ 3,783,158	$ 3,617,135