Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 5,269,175	$ 5,017,617
Other comprehensive (loss) income	(134,365)	25,289
Ending balance	5,285,027	4,902,985
AFS Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	52,247	(2,419)
Net unrealized gains (losses) arising during the period	(133,313)	28,530
Amounts reclassified from AOCI	(135)	(1,077)
Other comprehensive (loss) income	(133,448)	27,453
Ending balance	(81,201)	25,034
Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,230)	0
Net unrealized gains (losses) arising during the period	305	0
Amounts reclassified from AOCI	127	0
Other comprehensive (loss) income	432	0
Ending balance	(798)	0
Foreign Currency Translation Adjustments, Net of Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(6,692)	(15,989)
Net unrealized gains (losses) arising during the period	(1,349)	(2,164)
Amounts reclassified from AOCI	0	0
Other comprehensive (loss) income	(1,349)	(2,164)
Ending balance	(8,041)	(18,153)
AOCI, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	44,325	(18,408)
Net unrealized gains (losses) arising during the period	(134,357)	26,366
Amounts reclassified from AOCI	(8)	(1,077)
Other comprehensive (loss) income	(134,365)	25,289
Ending balance	$ (90,040)	$ 6,881