Accumulated Other Comprehensive Income (Loss) - Rollforward of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Changes in accumulated other comprehensive income by component
Balance	$ 5,655,833	$ 2,645,845	$ 5,727,241	$ 2,556,260
Total other comprehensive income (loss), net of tax	32,238	31,853	144,131	70,472
Balance	5,658,555	2,710,517	5,658,555	2,710,517
Total
Changes in accumulated other comprehensive income by component
Balance	166,170	5,481	54,277	(33,138)
Other comprehensive income (loss)	31,039	32,558	142,231	70,609
Amounts reclassified from accumulated other comprehensive income (loss)	1,199	(705)	1,900	(137)
Total other comprehensive income (loss), net of tax	32,238	31,853	144,131	70,472
Balance	198,408	37,334	198,408	37,334
Net Unrealized Gains (Losses) on Available-for-Sale Investment Securities and Interest-only Strips
Changes in accumulated other comprehensive income by component
Balance	171,945	9,346	56,098	(28,022)
Other comprehensive income (loss)	29,189	31,322	144,326	68,114
Amounts reclassified from accumulated other comprehensive income (loss)	1,208	(697)	1,918	(121)
Total other comprehensive income (loss), net of tax	30,397	30,625	146,244	67,993
Balance	202,342	39,971	202,342	39,971
Net Unrealized Gains (Losses) on Net Investment Hedges
Changes in accumulated other comprehensive income by component
Balance	20,281	12,678	9,800	14,986
Other comprehensive income (loss)	(4,557)	(2,179)	5,924	(4,487)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0	0
Total other comprehensive income (loss), net of tax	(4,557)	(2,179)	5,924	(4,487)
Balance	15,724	10,499	15,724	10,499
Foreign Currency Translation Adjustment
Changes in accumulated other comprehensive income by component
Balance	(26,123)	(16,644)	(11,697)	(20,211)
Other comprehensive income (loss)	6,407	3,415	(8,019)	6,982
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0	0
Total other comprehensive income (loss), net of tax	6,407	3,415	(8,019)	6,982
Balance	(19,716)	(13,229)	(19,716)	(13,229)
Recognized Postretirement Prior Service Cost
Changes in accumulated other comprehensive income by component
Balance	67	101	76	109
Other comprehensive income (loss)	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(9)	(8)	(18)	(16)
Total other comprehensive income (loss), net of tax	(9)	(8)	(18)	(16)
Balance	$ 58	$ 93	$ 58	$ 93