Parent Company Financial Statements - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities
Net income	$ 1,079,667	$ 1,066,246	$ 1,138,480
Adjustments to reconcile net income to net cash provided by operating activities
Provision for deferred income taxes	396,596	92,848	139,785
Loss on sale of assets	(67,759)	(6,859)	(127,890)
Net cash provided by operating activities	1,742,408	1,099,041	932,324
Cash flows from investing activities
Other, net	10,876	19,531	95,706
Net cash provided (used) by investing activities	7,714,282	(11,710,371)	(2,203,578)
Cash flows from financing activities
Payments on long-term borrowings	(8,912,474)	(426,275)	(261,212)
Dividends paid - common	(375,017)	(371,199)	(365,349)
Dividends paid - preferred	(81,270)	(70,234)	(53,450)
Proceeds from issuance of preferred stock		346,500
Other, net	69,766	88,565	137,967
Net cash provided (used) by financing activities	(9,462,007)	10,311,753	957,573
Net increase (decrease) in cash and cash equivalents	(5,317)	(299,577)	(313,681)
Cash and cash equivalents at beginning of year	1,373,357	1,672,934	1,986,615
Cash and cash equivalents at end of year	1,368,040	1,373,357	1,672,934
Supplemental disclosure of cash flow information
Interest received during the year	3,134,311	2,893,153	2,894,699
Interest paid during the year	400,329	257,553	301,734
Income taxes received during the year	378,660	411,912	389,008
Parent Company [Member]
Cash flows from operating activities
Net income	1,079,667	1,066,246	1,138,480
Adjustments to reconcile net income to net cash provided by operating activities
Equity in undistributed income of subsidiaries	(632,851)	(630,686)	(497,737)
Provision for deferred income taxes	(3,655)	(6,522)	1,535
Net change in accrued income and expense	21,780	23,419	31,979
Loss on sale of assets	119
Net cash provided by operating activities	465,060	452,457	674,257
Cash flows from investing activities
Proceeds from sales of investment securities	755
Investment in subsidiary			(140,000)
Other, net	14,038	10,721	3,295
Net cash provided (used) by investing activities	14,793	10,721	(136,705)
Cash flows from financing activities
Payments on long-term borrowings	(322,621)	(350,010)
Dividends paid - common	(375,017)	(371,199)	(365,349)
Dividends paid - preferred	(81,270)	(70,234)	(53,450)
Proceeds from issuance of preferred stock		346,500
Other, net	76,364	110,601	140,799
Net cash provided (used) by financing activities	(702,544)	(334,342)	(278,000)
Net increase (decrease) in cash and cash equivalents	(222,691)	128,836	259,552
Cash and cash equivalents at beginning of year	1,107,839	979,003	719,451
Cash and cash equivalents at end of year	885,148	1,107,839	979,003
Supplemental disclosure of cash flow information
Interest received during the year	1,905	2,094	2,224
Interest paid during the year	30,420	47,003	71,090
Income taxes received during the year	$ 16,696	$ 24,588	$ 45,237