Other Comprehensive Income (Loss) - Components of other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Before tax amount:
Unrealized holding gains (losses) arising during period, before tax	$ (88,034)	$ 108,011	$ (277,977)	$ 219,483
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), before tax	0	0	0	0
Less reclassification of net realized gains included in net income, before tax	(39,405)	22,381	(107,153)	28,387
Foreign currency translation adjustments, before tax	(12,701)	18,509	(11,269)	21,674
Other comprehensive income (loss), before tax	(61,330)	104,139	(182,093)	212,770
Tax expense (benefit):
Unrealized holding gains (losses) arising during period, tax	(2,763)	15,042	(26,029)	25,722
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), tax	0	0	0	0
Less reclassification of net realized gains included in net income, tax	(2,762)	5,157	(8,049)	6,119
Foreign currency translation adjustments, tax	(106)	212	44	253
Other comprehensive income (loss), tax	(107)	10,097	(17,936)	19,856
Net of tax amount:
Unrealized holding gains (losses) arising during period, net of tax	(85,271)	92,969	(251,948)	193,761
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), net of tax	0	0	0	0
Less reclassification of net realized gains included in net income, net of tax	(36,643)	17,224	(99,104)	22,268
Foreign currency translation adjustments, net of tax	(12,595)	18,297	(11,313)	21,421
Net current period other comprehensive income (loss)	$ (61,223)	$ 94,042	$ (164,157)	$ 192,914