Accumulated Other Comprehensive Income (Loss) - Rollforward of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in accumulated other comprehensive income by component
Balance, beginning of period	$ 5,727,241	$ 2,556,260	$ 2,680,584
Total other comprehensive income (loss), net of tax	128,396	87,415	(14,621)
Balance, end of period	5,689,297	5,727,241	2,556,260
Total
Changes in accumulated other comprehensive income by component
Balance, beginning of period	54,277	(33,138)	(18,517)
Other comprehensive income (loss)	127,230	87,024	(15,289)
Amounts reclassified from accumulated other comprehensive income (loss)	1,166	391	668
Total other comprehensive income (loss), net of tax	128,396	87,415	(14,621)
Balance, end of period	182,673	54,277	(33,138)
Net Unrealized Gains (Losses) on Available-for-Sale Investment Securities and Interest-only Strips
Changes in accumulated other comprehensive income by component
Balance, beginning of period	56,098	(28,022)	(16,353)
Other comprehensive income (loss)	125,189	83,696	(12,371)
Amounts reclassified from accumulated other comprehensive income (loss)	1,201	424	702
Total other comprehensive income (loss), net of tax	126,390	84,120	(11,669)
Balance, end of period	182,488	56,098	(28,022)
Net Unrealized Gains (Losses) on Net Investment Hedges
Changes in accumulated other comprehensive income by component
Balance, beginning of period	9,800	14,986	4,536
Other comprehensive income (loss)	(4,195)	(5,186)	10,450
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0
Total other comprehensive income (loss), net of tax	(4,195)	(5,186)	10,450
Balance, end of period	5,605	9,800	14,986
Foreign Currency Translation Adjustment
Changes in accumulated other comprehensive income by component
Balance, beginning of period	(11,697)	(20,211)	(6,843)
Other comprehensive income (loss)	5,894	8,514	(13,368)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0
Total other comprehensive income (loss), net of tax	5,894	8,514	(13,368)
Balance, end of period	(5,803)	(11,697)	(20,211)
Recognized Postretirement Prior Service Cost
Changes in accumulated other comprehensive income by component
Balance, beginning of period	76	109	143
Other comprehensive income (loss)	342	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	(35)	(33)	(34)
Total other comprehensive income (loss), net of tax	307	(33)	(34)
Balance, end of period	$ 383	$ 76	$ 109