Parent Company Financial Statements - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net income	$ 1,408,306	$ 1,315,114	$ 1,079,667
Adjustments to reconcile net income to net cash provided by operating activities
Benefit (provision) for deferred income taxes	400,790	174,013	396,596
Net gain on sales of assets	(53,467)	(63,222)	(67,759)
Net cash provided by operating activities	2,781,935	1,183,411	1,742,408
Cash flows from investing activities
Other, net	19,825	277,961	10,876
Net cash provided by investing activities	3,394,925	(720,768)	7,714,282
Cash flows from financing activities
Payments on long-term borrowings	(3,433,440)	(1,119,898)	(8,912,474)
Purchases of treasury stock	(1,205,905)	(641,334)
Dividends paid — common	(457,402)	(441,891)	(375,017)
Dividends paid — preferred	(72,734)	(81,270)	(81,270)
Redemption of Series D preferred stock		(500,000)
Other, net	10,675	161,691	69,766
Net cash used by financing activities	(6,076,521)	(510,134)	(9,462,007)
Net increase (decrease) in cash and cash equivalents	100,339	(47,491)	(5,317)
Cash and cash equivalents at beginning of year	1,320,549	1,368,040	1,373,357
Cash and cash equivalents at end of year	1,420,888	1,320,549	1,368,040
Supplemental disclosure of cash flow information
Interest received during the year	4,155,723	3,903,374	3,134,311
Interest paid during the year	405,290	498,951	400,329
Income taxes received during the year	494,205	276,866	378,660
Parent Company [Member]
Cash flows from operating activities
Net income	1,408,306	1,315,114	1,079,667
Adjustments to reconcile net income to net cash provided by operating activities
Equity in undistributed income of subsidiaries	126,765	586,587	(632,851)
Benefit (provision) for deferred income taxes	4,543	(3,157)	(3,655)
Net change in accrued income and expense	(170)	12,898	21,780
Net gain on sales of assets	(2,995)	(2,342)	119
Net cash provided by operating activities	1,536,449	1,909,100	465,060
Cash flows from investing activities
Proceeds from sales or maturities of investment securities		51	755
Other, net	12,407	13,619	14,038
Net cash provided by investing activities	12,407	13,670	14,793
Cash flows from financing activities
Payments on long-term borrowings			(322,621)
Purchases of treasury stock	(1,205,905)	(641,334)
Dividends paid — common	(457,402)	(441,891)	(375,017)
Dividends paid — preferred	(72,734)	(81,270)	(81,270)
Redemption of Series D preferred stock		(500,000)
Proceeds from issuance of Series F preferred stock		495,000
Other, net	34,524	143,764	76,364
Net cash used by financing activities	(1,701,517)	(1,025,731)	(702,544)
Net increase (decrease) in cash and cash equivalents	(152,661)	897,039	(222,691)
Cash and cash equivalents at beginning of year	1,782,187	885,148	1,107,839
Cash and cash equivalents at end of year	1,629,526	1,782,187	885,148
Supplemental disclosure of cash flow information
Interest received during the year	2,313	1,931	1,905
Interest paid during the year	18,498	15,918	30,420
Income taxes received during the year	$ 21,740	$ 8,877	$ 16,696