Other Comprehensive Income (Loss) - Components of other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Before tax amount:
Unrealized holding gains (losses) arising during period, before tax	$ 108,011	$ 117,904	$ 219,483	$ 270,078
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), before tax	0	(52)	0	(150)
Less reclassification of net realized gains included in net income, before tax	22,381	26,009	28,387	62,959
Foreign currency translation adjustments, before tax	18,509	(18,186)	21,674	(326)
Other comprehensive income (loss), before tax	104,139	73,657	212,770	206,643
Tax expense (benefit):
Unrealized holding gains (losses) arising during period, tax	15,042	15,444	25,722	34,637
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	5,157	3,915	6,119	8,642
Foreign currency translation adjustments, tax	212	(35)	253	512
Other comprehensive income (loss), tax	10,097	11,494	19,856	26,507
Net of tax amount:
Unrealized holding gains (losses) arising during period, net of tax	92,969	102,460	193,761	235,441
Portion of other-than-temporary impairment losses recognized in other comprehensive income (loss), net of tax	0	(52)	0	(150)
Less reclassification of net realized gains included in net income, net of tax	17,224	22,094	22,268	54,317
Foreign currency translation adjustments, net of tax	18,297	(18,151)	21,421	(838)
Net current period other comprehensive income (loss)	$ 94,042	$ 62,163	$ 192,914	$ 180,136