Condensed Statements of Cash Flow (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CASH FLOWS FROM OPERATING ACTIVITIES:
Net (loss) income	$ (47,156)	$ 485,397	$ 475,547
Change in other assets	(196,899)	105,575	(92,089)
Change in other liabilities	15,425	(16,020)	49,442
CASH FLOWS FROM FINANCING ACTIVITIES:
Treasury stock purchases	(7,020)	(7,283)	(5,319)
Cash dividends paid on common stock	(453,981)	(442,204)	(440,308)
Net cash received from exercise of stock options		60	326
Proceeds from follow-on common stock offering ,net	629,682
Net (decrease) increase in cash and cash equivalents	(26,476)	(80,400)	(1,782,708)
Cash and cash equivalents at beginning of year	564,150	644,550	2,427,258
Cash and cash equivalents at end of year	537,674	564,150	644,550
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES:
Net (loss) income	(47,156)	485,397	475,547
Change in other assets	(2,253)	293	(3,841)
Change in other liabilities	22,236	(2,807)	6,342
Other, net	32,955	30,739	24,135
Equity in overdistributed (underdistributed) earnings of subsidiaries	366,175	(98,701)	(46,041)
Net cash provided by operating activities	371,957	414,921	456,142
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from sales and repayments of securities		566	151
Change in receivable from subsidiaries, net	224	(2,707)	1,428
Investment in subsidiaries	(560,000)
Net cash (used in) provided by investing activities	(559,776)	(2,141)	1,579
CASH FLOWS FROM FINANCING ACTIVITIES:
Treasury stock purchases	(7,020)	(7,283)	(5,319)
Cash dividends paid on common stock	(453,981)	(442,204)	(440,308)
Net cash received from exercise of stock options		60	326
Proceeds from follow-on common stock offering ,net	629,682
Net cash provided by (used in) financing activities	168,681	(449,427)	(445,301)
Net (decrease) increase in cash and cash equivalents	(19,138)	(36,647)	12,420
Cash and cash equivalents at beginning of year	89,518	126,165	113,745
Cash and cash equivalents at end of year	$ 70,380	$ 89,518	$ 126,165