Income Taxes - Summary of Reconciliation of the Income Tax (Benefit) Expense at the Statutory Income Tax Rate and the Effective Tax Rate for Income from Continuing Operations before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income from continuing operations (before-tax)	$ 219,108	$ 141,430	$ 51,138
Income tax at the US statutory federal rate (35%)	76,688	49,501	17,898
Increase (decrease) resulting from:
Rate differential	1,434	5,047	1,145
State income taxes, net of federal benefit	66	(601)	(337)
Dividends paid deduction	(84,927)	(57,109)	(27,873)
Equity Investments	4,297
Change in valuation allowance	(6,104)	6,174	8,988
Other items, net	1,716	(1,509)	519
Income tax (benefit) expense	$ (6,830)	$ 1,503	$ 340