Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Accumulated other comprehensive income (loss)
Beginning balance	$ 1,788	$ 1,570	$ 1,570
Other comprehensive income, net of tax	30	16
Ending balance	1,842	1,574	1,788
Investment Securities
Accumulated other comprehensive income (loss)
Beginning balance	1	(47)	(47)
Unrealized gain	45	21
Less: Tax provision	11	5
Net unrealized loss, net of tax	34	16
Reclassification out of AOCI	0	0
Less: Tax provision	0	0
Net unrealized gain reclassified out of AOCI	0	0
Other comprehensive income, net of tax	34	16
Ending balance	35	(31)	1
Cash Flow Hedges
Accumulated other comprehensive income (loss)
Beginning balance	0	0	0
Unrealized gain	(5)	0
Less: Tax provision	(1)	0
Net unrealized loss, net of tax	(4)	0
Reclassification out of AOCI	0	0
Less: Tax provision	0	0
Net unrealized gain reclassified out of AOCI	0	0
Other comprehensive income, net of tax	(4)	0
Ending balance	$ (4)	$ 0	$ 0