Schedule I - Condensed Statements of Cash Flows (Parent Company Only) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
NET INCOME	$ 113,396	$ 107,074	$ 146,963	$ 159,692	$ (83,558)	$ 109,210	$ 120,378	$ 107,553	$ 527,125	$ 253,583	$ 387,698
CASH FLOWS FROM INVESTING ACTIVITIES:
Net cash used in investing activities									(46,209)	(26,920)	(37,653)
CASH FLOWS FROM FINANCING ACTIVITIES:
Class A common stock dividends									(359,639)	(341,450)	(336,138)
Cash and cash equivalents, January 1				1,483,836				1,158,785	1,483,836	1,158,785
Cash and cash equivalents, December 31	1,246,537				1,483,836				1,246,537	1,483,836	1,158,785
Parent Company [Member]
CASH FLOWS FROM OPERATING ACTIVITIES:
NET INCOME									527,125	253,583	387,698
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in earnings of subsidiaries									(392,574)	(145,008)	(326,366)
Dividends received from subsidiaries									189,786	63,822
Changes in due to/from subsidiaries									35,504	169,056	273,051
Changes in other operating assets and liabilities									13	27	(233)
Net cash provided by operating activities									359,854	341,480	334,150
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									(359,639)	(341,450)	(336,138)
Net cash used in financing activities									(359,639)	(341,450)	(336,138)
Net increase (decrease) in cash and cash equivalents									215	5	(1,988)
Cash and cash equivalents, January 1				$ 3,921				$ 3,916	3,921	3,916	5,904
Cash and cash equivalents, December 31	$ 4,136				$ 3,921				$ 4,136	$ 3,921	$ 3,916