Parent Company Financial Statements - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net income	$ 1,315,114	$ 1,079,667	$ 1,066,246
Adjustments to reconcile net income to net cash provided by operating activities
Provision for deferred income taxes	174,013	396,596	92,848
Net gain on sales of assets	(63,222)	(67,759)	(6,859)
Net cash provided by operating activities	1,183,411	1,742,408	1,099,041
Cash flows from investing activities
Other, net	277,961	10,876	19,531
Net cash provided by investing activities	(720,768)	7,714,282	(11,710,371)
Cash flows from financing activities
Payments on long-term borrowings	(1,119,898)	(8,912,474)	(426,275)
Purchases of treasury stock	(641,334)
Dividends paid — common	(441,891)	(375,017)	(371,199)
Dividends paid — preferred	(81,270)	(81,270)	(70,234)
Redemption of Series D preferred stock	(500,000)
Proceeds from issuance of preferred stock	495,000		346,500
Other, net	161,691	69,766	88,565
Net cash provided (used) by financing activities	(510,134)	(9,462,007)	10,311,753
Net increase (decrease) in cash and cash equivalents	(47,491)	(5,317)	(299,577)
Cash and cash equivalents at beginning of year	1,368,040	1,373,357	1,672,934
Cash and cash equivalents at end of year	1,320,549	1,368,040	1,373,357
Supplemental disclosure of cash flow information
Interest received during the year	3,903,374	3,134,311	2,893,153
Interest paid during the year	498,951	400,329	257,553
Income taxes received during the year	276,866	378,660	411,912
Parent Company [Member]
Cash flows from operating activities
Net income	1,315,114	1,079,667	1,066,246
Adjustments to reconcile net income to net cash provided by operating activities
Equity in undistributed income of subsidiaries	586,587	(632,851)	(630,686)
Provision for deferred income taxes	(3,157)	(3,655)	(6,522)
Net change in accrued income and expense	12,898	21,780	23,419
Net gain on sales of assets	(2,342)	119
Net cash provided by operating activities	1,909,100	465,060	452,457
Cash flows from investing activities
Proceeds from sales or maturities of investment securities	51	755
Other, net	13,619	14,038	10,721
Net cash provided by investing activities	13,670	14,793	10,721
Cash flows from financing activities
Payments on long-term borrowings		(322,621)	(350,010)
Purchases of treasury stock	(641,334)
Dividends paid — common	(441,891)	(375,017)	(371,199)
Dividends paid — preferred	(81,270)	(81,270)	(70,234)
Redemption of Series D preferred stock	(500,000)
Proceeds from issuance of preferred stock	495,000		346,500
Other, net	143,764	76,364	110,601
Net cash provided (used) by financing activities	(1,025,731)	(702,544)	(334,342)
Net increase (decrease) in cash and cash equivalents	897,039	(222,691)	128,836
Cash and cash equivalents at beginning of year	885,148	1,107,839	979,003
Cash and cash equivalents at end of year	1,782,187	885,148	1,107,839
Supplemental disclosure of cash flow information
Interest received during the year	1,931	1,905	2,094
Interest paid during the year	15,918	30,420	47,003
Income taxes received during the year	$ 8,877	$ 16,696	$ 24,588