Accumulated Other Comprehensive Income (Loss) - Rollforward of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Changes in accumulated other comprehensive income by component
Balance	$ 5,689,297	$ 5,727,241
Total other comprehensive income (loss), net of tax	(180,019)	111,893
Balance	5,591,771	5,655,833
Total
Changes in accumulated other comprehensive income by component
Balance	182,673	54,277
Other comprehensive income (loss)	(180,072)	111,192
Amounts reclassified from accumulated other comprehensive income (loss)	53	701
Total other comprehensive income (loss), net of tax	(180,019)	111,893
Balance	2,654	166,170
Net Unrealized Gains (Losses) on Available-for-Sale Investment Securities and Interest-only Strips
Changes in accumulated other comprehensive income by component
Balance	182,488	56,098
Other comprehensive income (loss)	(180,683)	115,137
Amounts reclassified from accumulated other comprehensive income (loss)	71	710
Total other comprehensive income (loss), net of tax	(180,612)	115,847
Balance	1,876	171,945
Net Unrealized Gains (Losses) on Net Investment Hedges
Changes in accumulated other comprehensive income by component
Balance	5,605	9,800
Other comprehensive income (loss)	(1,559)	10,481
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Total other comprehensive income (loss), net of tax	(1,559)	10,481
Balance	4,046	20,281
Foreign Currency Translation Adjustment
Changes in accumulated other comprehensive income by component
Balance	(5,803)	(11,697)
Other comprehensive income (loss)	2,170	(14,426)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Total other comprehensive income (loss), net of tax	2,170	(14,426)
Balance	(3,633)	(26,123)
Recognized Postretirement Prior Service Cost
Changes in accumulated other comprehensive income by component
Balance	383	76
Other comprehensive income (loss)	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(18)	(9)
Total other comprehensive income (loss), net of tax	(18)	(9)
Balance	$ 365	$ 67