Parent Company Financial Statements - Statements of Cash Flows of Parent Company Only (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities
Net income	$ 72	$ 83	$ 84	$ 47	$ 95	$ 103	$ 95	$ 94	$ 286	$ 387	$ 373
Other, net									(177)	(225)	(27)
Net cash flows provided by operating activities									113	259	611
Investing Activities
Net cash flows used in investing activities									(1,822)	(1,131)	(1,920)
Financing Activities
Decrease in long-term debt									(574)	(239)	(77)
Increase in long-term debt									328	954	37
Other, net									17	12	14
Preferred stock									(8)	(8)	(8)
Common stock									(157)	(157)	(157)
Net cash flows provided by financing activities									2,493	983	1,318
Net Increase (Decrease) in Cash and Cash Equivalents									784	111	9
Cash and cash equivalents at beginning of year				599				488	599	488	479
Cash and Cash Equivalents at End of Year	1,383				599				1,383	599	488
Interest									216	329	230
Parent Company
Operating Activities
Net income									286	387	373
Undistributed earnings from subsidiaries	(12)				(236)				(12)	(236)	(222)
Other, net									13	2	(13)
Net cash flows provided by operating activities									287	153	138
Investing Activities
Proceeds from sale of securities available for sale									0	0	1
Net decrease in advances to subsidiaries									0	0	20
Payment for further investment in subsidiaries									(270)	(47)	(22)
Net cash received in business combinations									0	0	123
Net cash flows used in investing activities									(270)	(47)	122
Financing Activities
Net decrease in advance from affiliate									0	0	(19)
Net decrease in short-term borrowings									0	0	(1)
Decrease in long-term debt									(4)	(77)	(2)
Increase in long-term debt									302	121	1
Other, net									(21)	12	14
Preferred stock									(8)	(8)	(8)
Common stock									(157)	(157)	(157)
Net cash flows provided by financing activities									112	(109)	(172)
Net Increase (Decrease) in Cash and Cash Equivalents									129	(3)	88
Cash and cash equivalents at beginning of year				$ 251				$ 254	251	254	166
Cash and Cash Equivalents at End of Year	$ 380				$ 251				380	251	254
Interest									$ 26	$ 20	$ 17