Other Comprehensive Income (Loss) - Components of other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Before tax amount:
Unrealized holding gains (losses) arising during period, before tax	$ 248,074	$ (88,034)	$ 503,064	$ (277,977)
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	59,760	(39,405)	70,160	(107,153)
Foreign currency translation adjustments, before tax	4,409	(12,701)	10,053	(11,269)
Other comprehensive income (loss), before tax	192,723	(61,330)	442,957	(182,093)
Tax expense (benefit):
Unrealized holding gains (losses) arising during period, tax	25,601	(2,763)	54,704	(26,029)
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	4,415	(2,762)	4,594	(8,049)
Foreign currency translation adjustments, tax	142	(106)	270	44
Other comprehensive income (loss), tax	21,328	(107)	50,380	(17,936)
Net of tax amount:
Unrealized holding gains (losses) arising during period, net of tax	222,473	(85,271)	448,360	(251,948)
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	55,345	(36,643)	65,566	(99,104)
Foreign currency translation adjustments, net of tax	4,267	(12,595)	9,783	(11,313)
Net current period other comprehensive income (loss)	$ 171,395	$ (61,223)	$ 392,577	$ (164,157)