Schedule I - Condensed Statements of Cash Flows (Parent Company Only) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
NET INCOME	$ (83,558)	$ 109,210	$ 120,378	$ 107,553	$ 127,982	$ 112,536	$ 80,357	$ 66,823	$ 253,583	$ 387,698	$ 986,373
Adjustments to reconcile net income to net cash provided by operating activities:
Net cash provided by operating activities									1,015,442	601,287	887,296
CASH FLOWS FROM FINANCING ACTIVITIES:
Class A common stock dividends									341,450	336,138	290,684
Net cash used in financing activities									(740,393)	(486,952)	(746,804)
NET INCREASE IN CASH AND CASH EQUIVALENTS									325,051	26,702	65,503
CASH AND CASH EQUIVALENTS—January 1				1,158,785				1,132,083	1,158,785	1,132,083	1,066,580
CASH AND CASH EQUIVALENTS—December 31	1,483,836				1,158,785				1,483,836	1,158,785	1,132,083
Parent Company [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
NET INCOME									253,583	387,698	986,373
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in earnings of subsidiaries									(145,008)	(326,366)	(931,036)
Dividends received from subsidiaries									63,822
Changes in due to/from subsidiaries									169,056	273,051	233,519
Changes in other operating assets and liabilities									27	(233)	250
Net cash provided by operating activities									341,480	334,150	289,106
CASH FLOWS FROM INVESTING ACTIVITIES:
Capital contribution to subsidiaries									(25)
Net cash used in investing activities									(25)
CASH FLOWS FROM FINANCING ACTIVITIES:
Class A common stock dividends									(341,450)	(336,138)	(290,684)
Net cash used in financing activities									(341,450)	(336,138)	(290,684)
NET INCREASE IN CASH AND CASH EQUIVALENTS									5	(1,988)	(1,578)
CASH AND CASH EQUIVALENTS—January 1				$ 3,916				$ 5,904	3,916	5,904	7,482
CASH AND CASH EQUIVALENTS—December 31	$ 3,921				$ 3,916				$ 3,921	$ 3,916	$ 5,904