Schedule II. Condensed Financial Information of Registrant (Notes To Condensed Financial Statements) (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Dividends from subsidiaries	$ 454,000	$ 438,000	$ 466,000
Stock-based compensation not involving cash	37,034	26,326	28,664
Interest paid	82,494	81,338	74,792
Income taxes received	$ 74,379	$ 79,790	110,650
Preferred stock, shares outstanding	0	0
Parent Company
Condensed Financial Statements, Captions [Line Items]
Dividends from subsidiaries	$ 453,904	$ 437,566	466,416
Stock-based compensation not involving cash	37,034	26,326	28,664
Borrowed money from affiliate	0	0	56,503
Investment in subsidiaries	317,027	0	39,206
Borrowed money from affiliate	0	0	17,297
Interest paid	86,606	84,952	77,920
Income taxes received	$ (19,961)	$ (20,838)	$ (22,009)
Preferred stock, liquidation distribution available to stockholders, per share in thousands	$ 1,000
Preferred stock, liquidating distribution legally available, aggregate value	$ 351,000
Cumulative Series A Preferred Stock | Parent Company
Condensed Financial Statements, Captions [Line Items]
Preferred stock, shares issued	351,000
Preferred stock, shares outstanding	351,000
6.50% Cumulative Preferred Stock, Series A | Parent Company
Condensed Financial Statements, Captions [Line Items]
Preferred stock, shares issued	280,000
Preferred stock, shares outstanding	280,000
Preferred stock, dividend rate, percentage	6.50%
7.15% Cumulative Preferred Stock, Series A | Parent Company
Condensed Financial Statements, Captions [Line Items]
Preferred stock, shares issued	71,000
Preferred stock, shares outstanding	71,000
Preferred stock, dividend rate, percentage	7.15%