Condensed Statements of Cash Flow (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)		$ 466,201	$ 495,401	$ (47,156)
Change in other assets		451,873	326,790	(196,899)
Change in other liabilities		23,329	(4,336)	15,425
CASH FLOWS FROM FINANCING ACTIVITIES:
Treasury stock purchases	[1]	(18,463)	(8,677)	(7,020)
Proceeds from issuance of preferred stock		502,840
Proceeds from follow-on common stock offering, net				629,682
Net increase (decrease) in cash and cash equivalents		1,970,319	20,176	(26,476)
Cash and cash equivalents at beginning of year		557,850	537,674	564,150
Cash and cash equivalents at end of year		2,528,169	557,850	537,674
Parent Company
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)		466,201	495,401	(47,156)
Change in other assets		10,122	316	(2,253)
Change in other liabilities		(36,226)	(2,252)	22,236
Other, net		36,330	33,333	32,955
Equity in (underdistributed) overdistributed earnings of subsidiaries		(162,316)	(193,760)	366,175
Net cash provided by operating activities		314,110	333,038	371,957
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from sales and repayments of securities		2,000
Change in receivable from subsidiaries, net		3,089	(204)	224
Investment in subsidiaries		(420,000)		(560,000)
Net cash used in investing activities		(414,911)	(204)	(559,776)
CASH FLOWS FROM FINANCING ACTIVITIES:
Treasury stock purchases		(18,463)	(8,677)	(7,020)
Cash dividends paid on common and preferred stock		(356,768)	(330,810)	(453,981)
Proceeds from issuance of preferred stock		502,840
Proceeds from follow-on common stock offering, net				629,682
Net cash provided by (used in) financing activities		127,609	(339,487)	168,681
Net increase (decrease) in cash and cash equivalents		26,809	(6,653)	(19,138)
Cash and cash equivalents at beginning of year		63,727	70,380	89,518
Cash and cash equivalents at end of year		$ 90,536	$ 63,727	$ 70,380

[1] See Note 2, "Summary of Significant Accounting Policies" for a discussion of the Company's adoption of Accounting Standards Update No. 2016-09.