Parent Company Financial Statements - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net income	$ 1,929,149	$ 1,918,080	$ 1,408,306
Adjustments to reconcile net income to net cash provided by operating activities
Provision for deferred income taxes	57,548	15,857	400,790
Gain on sale of assets	31,526	(23,503)	(53,467)
Net cash provided by operating activities	2,357,555	2,089,852	2,781,935
Cash flows from investing activities
Other, net	(195,921)	47,904	19,825
Cash flows from financing activities
Purchases of treasury stock	(1,349,785)	(2,194,396)	(1,205,905)
Dividends paid — common	(552,138)	(510,382)	(457,402)
Dividends paid — preferred	(67,454)	(72,521)	(72,734)
Proceeds from long-term borrowings		1,773,189	2,145,950
Redemption of Series A and Series C preferred stock	(381,500)
Other, net	(25,393)	17,167	10,675
Supplemental disclosure of cash flow information
Interest received during the year	4,892,301	4,568,991	4,155,723
Income taxes received during the year	320,513	375,116	494,205
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	1,929,149	1,918,080	1,408,306
Adjustments to reconcile net income to net cash provided by operating activities
Equity in undistributed income of subsidiaries	70,946	(693,965)	126,765
Provision for deferred income taxes	5,263	4,949	4,543
Net change in accrued income and expense	(34,525)	(8,242)	(170)
Gain on sale of assets			(2,995)
Net cash provided by operating activities	1,970,833	1,220,822	1,536,449
Cash flows from investing activities
Proceeds from sales or maturities of investment securities	100
Other, net	51,235	29,933	12,407
Net cash provided by investing activities	51,335	29,933	12,407
Cash flows from financing activities
Purchases of treasury stock	(1,349,785)	(2,194,396)	(1,205,905)
Dividends paid — common	(552,138)	(510,382)	(457,402)
Dividends paid — preferred	(67,454)	(72,521)	(72,734)
Proceeds from long-term borrowings		748,595
Redemption of Series A and Series C preferred stock	(381,500)
Other, net	(4,431)	45,913	34,524
Net cash used by financing activities	(1,959,308)	(1,982,791)	(1,701,517)
Net increase (decrease) in cash and cash equivalents	62,860	(732,036)	(152,661)
Cash and cash equivalents at beginning of year	897,490	1,629,526	1,782,187
Cash and cash equivalents at end of year	960,350	897,490	1,629,526
Supplemental disclosure of cash flow information
Interest received during the year	1,752	2,219	2,313
Interest paid during the year	49,451	17,482	18,498
Income taxes received during the year	6,251	$ 6,362	$ 21,740
Parent Company [Member] | Series G Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of Series G preferred stock	$ 396,000