Parent Company Financial Statements - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net income	$ 1,918,080	$ 1,408,306	$ 1,315,114
Adjustments to reconcile net income to net cash provided by operating activities
Benefit (provision) for deferred income taxes	15,857	400,790	174,013
Net gain on sales of assets	(23,503)	(53,467)	(63,222)
Net cash provided by operating activities	2,089,852	2,781,935	1,183,411
Cash flows from investing activities
Other, net	47,904	19,825	277,961
Cash flows from financing activities
Purchases of treasury stock	(2,194,396)	(1,205,905)	(641,334)
Dividends paid — common	(510,382)	(457,402)	(441,891)
Dividends paid — preferred	(72,521)	(72,734)	(81,270)
Redemption of Series D preferred stock			(500,000)
Other, net	17,167	10,675	161,691
Proceeds from long-term borrowings	1,773,189	2,145,950
Supplemental disclosure of cash flow information
Interest received during the year	4,568,991	4,155,723	3,903,374
Income taxes received during the year	375,116	494,205	276,866
Parent Company [Member]
Cash flows from operating activities
Net income	1,918,080	1,408,306	1,315,114
Adjustments to reconcile net income to net cash provided by operating activities
Equity in undistributed income of subsidiaries	(693,965)	126,765	586,587
Benefit (provision) for deferred income taxes	4,949	4,543	(3,157)
Net change in accrued income and expense	(8,242)	(170)	12,898
Net gain on sales of assets		(2,995)	(2,342)
Net cash provided by operating activities	1,220,822	1,536,449	1,909,100
Cash flows from investing activities
Proceeds from sales or maturities of investment securities			51
Other, net	29,933	12,407	13,619
Net cash provided by investing activities	29,933	12,407	13,670
Cash flows from financing activities
Purchases of treasury stock	(2,194,396)	(1,205,905)	(641,334)
Dividends paid — common	(510,382)	(457,402)	(441,891)
Dividends paid — preferred	(72,521)	(72,734)	(81,270)
Redemption of Series D preferred stock			(500,000)
Proceeds from issuance of Series F preferred stock			495,000
Other, net	45,913	34,524	143,764
Net cash used by financing activities	(1,982,791)	(1,701,517)	(1,025,731)
Proceeds from long-term borrowings	748,595
Net increase (decrease) in cash and cash equivalents	(732,036)	(152,661)	897,039
Cash and cash equivalents at beginning of year	1,629,526	1,782,187	885,148
Cash and cash equivalents at end of year	897,490	1,629,526	1,782,187
Supplemental disclosure of cash flow information
Interest received during the year	2,219	2,313	1,931
Interest paid during the year	17,482	18,498	15,918
Income taxes received during the year	$ 6,362	$ 21,740	$ 8,877