Parent Company Financial Statements - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net income	$ 1,353,152	$ 1,929,149	$ 1,918,080
Adjustments to reconcile net income to net cash provided by operating activities
Provision for deferred income taxes	(31,291)	57,548	15,857
Net cash provided by operating activities	789,187	2,357,555	2,089,852
Cash flows from investing activities
Other, net	67,411	(195,921)	47,904
Cash flows from financing activities
Purchases of treasury stock	(373,750)	(1,349,785)	(2,194,396)
Dividends paid — common	(568,112)	(552,138)	(510,382)
Dividends paid — preferred	(68,256)	(67,454)	(72,521)
Proceeds from long-term borrowings			1,773,189
Redemption of Series A and Series C preferred stock		(381,500)
Other, net	(11,413)	(25,393)	17,167
Supplemental disclosure of cash flow information
Interest received during the year	4,135,990	4,892,301	4,568,991
Income taxes received during the year	275,558	320,513	375,116
Series G Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of Series G preferred stock		396,000
Parent Company [Member]
Cash flows from operating activities
Net income	1,353,152	1,929,149	1,918,080
Adjustments to reconcile net income to net cash provided by operating activities
Equity in undistributed income of subsidiaries	(650,246)	70,946	(693,965)
Provision for deferred income taxes	1,079	5,263	4,949
Net change in accrued income and expense	(24,206)	(34,525)	(8,242)
Net cash provided by operating activities	679,779	1,970,833	1,220,822
Cash flows from investing activities
Proceeds from sales or maturities of investment securities		100
Other, net	176,050	51,235	29,933
Net cash provided by investing activities	176,050	51,335	29,933
Cash flows from financing activities
Purchases of treasury stock	(373,750)	(1,349,785)	(2,194,396)
Dividends paid — common	(568,112)	(552,138)	(510,382)
Dividends paid — preferred	(68,256)	(67,454)	(72,521)
Proceeds from long-term borrowings			748,595
Redemption of Series A and Series C preferred stock		(381,500)
Other, net	(5,992)	(4,431)	45,913
Net cash used by financing activities	(1,016,110)	(1,959,308)	(1,982,791)
Net increase (decrease) in cash and cash equivalents	(160,281)	62,860	(732,036)
Cash and cash equivalents at beginning of year	960,350	897,490	1,629,526
Cash and cash equivalents at end of year	800,069	960,350	897,490
Supplemental disclosure of cash flow information
Interest received during the year	1,493	1,752	2,219
Interest paid during the year	30,913	49,451	17,482
Income taxes received during the year	$ 11,528	6,251	$ 6,362
Parent Company [Member] | Series G Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of Series G preferred stock		$ 396,000