Income Taxes - Reconciliation Between Cash Dividends Paid and Dividends Paid Deduction (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Dividends paid	$ 293,031	$ 264,243	$ 239,294
Plus: Deemed dividends on convertible debt	0	14,159	12,026
Less: Dividends designated to prior year	(5,594)	(5,594)	(6,608)
Plus: Dividends designated from the following year	5,594	5,594	5,594
Less: Return of capital	(145,572)	(146,466)	(172,328)
Dividends paid deduction	$ 147,459	$ 131,936	$ 77,978