Accumulated Other Comprehensive Income (Loss) - Rollforward of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Changes in accumulated other comprehensive income by component
Balance	$ 5,727,241	$ 2,556,260
Total other comprehensive income (loss), net of tax	111,893	38,619
Balance	5,655,833	2,645,845
Total
Changes in accumulated other comprehensive income by component
Balance	54,277	(33,138)
Other comprehensive income (loss)	111,192	38,051
Amounts reclassified from accumulated other comprehensive income (loss)	701	568
Total other comprehensive income (loss), net of tax	111,893	38,619
Balance	166,170	5,481
Net Unrealized Gains (Losses) on Available-for-Sale Investment Securities and Interest-only Strips
Changes in accumulated other comprehensive income by component
Balance	56,098	(28,022)
Other comprehensive income (loss)	115,137	36,792
Amounts reclassified from accumulated other comprehensive income (loss)	710	576
Total other comprehensive income (loss), net of tax	115,847	37,368
Balance	171,945	9,346
Net Unrealized Gains (Losses) on Net Investment Hedges
Changes in accumulated other comprehensive income by component
Balance	9,800	14,986
Other comprehensive income (loss)	10,481	(2,308)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Total other comprehensive income (loss), net of tax	10,481	(2,308)
Balance	20,281	12,678
Foreign Currency Translation Adjustment
Changes in accumulated other comprehensive income by component
Balance	(11,697)	(20,211)
Other comprehensive income (loss)	(14,426)	3,567
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Total other comprehensive income (loss), net of tax	(14,426)	3,567
Balance	(26,123)	(16,644)
Recognized Postretirement Prior Service Cost
Changes in accumulated other comprehensive income by component
Balance	76	109
Other comprehensive income (loss)	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(9)	(8)
Total other comprehensive income (loss), net of tax	(9)	(8)
Balance	$ 67	$ 101