Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Changes in accumulated other comprehensive income by component
Balance	$ 2,710,517	$ 2,504,579	$ 2,556,260	$ 2,680,584
Total other comprehensive income (loss), net of tax	18,894	(12,448)	89,366	(46,742)
Balance	5,693,417	2,528,012	5,693,417	2,528,012
Total
Changes in accumulated other comprehensive income by component
Balance	37,334	(52,811)	(33,138)	(18,517)
Other comprehensive income (loss)	18,303	(12,575)	88,912	(47,490)
Amounts reclassified from accumulated other comprehensive income (loss)	(591)	(127)	(454)	(748)
Total other comprehensive income (loss), net of tax	18,894	(12,448)	89,366	(46,742)
Balance	56,228	(65,259)	56,228	(65,259)
Net Unrealized Gains (Losses) on Available-for-Sale Investment Securities and Interest-only Strips
Changes in accumulated other comprehensive income by component
Balance	39,971	(48,978)	(28,022)	(16,353)
Other comprehensive income (loss)	18,630	(13,570)	86,744	(46,833)
Amounts reclassified from accumulated other comprehensive income (loss)	(600)	(136)	(479)	(774)
Total other comprehensive income (loss), net of tax	19,230	(13,434)	87,223	(46,059)
Balance	59,201	(62,412)	59,201	(62,412)
Net Unrealized Gains (Losses) on Net Investment Hedges
Changes in accumulated other comprehensive income by component
Balance	10,499	9,919	14,986	4,536
Other comprehensive income (loss)	1,641	(1,904)	(2,846)	3,479
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0	0
Total other comprehensive income (loss), net of tax	1,641	(1,904)	(2,846)	3,479
Balance	12,140	8,015	12,140	8,015
Foreign Currency Translation Adjustment
Changes in accumulated other comprehensive income by component
Balance	(13,229)	(13,878)	(20,211)	(6,843)
Other comprehensive income (loss)	(1,968)	2,899	5,014	(4,136)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0	0
Total other comprehensive income (loss), net of tax	(1,968)	2,899	5,014	(4,136)
Balance	(15,197)	(10,979)	(15,197)	(10,979)
Recognized Postretirement Prior Service Cost
Changes in accumulated other comprehensive income by component
Balance	93	126	109	143
Other comprehensive income (loss)	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss)	9	9	25	26
Total other comprehensive income (loss), net of tax	(9)	(9)	(25)	(26)
Balance	$ 84	$ 117	$ 84	$ 117