Income Taxes - Summary of Reconciliation of the Income Tax Expense (Benefit) 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, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income from continuing operations (before-tax)	$ 293,919	$ 219,108	$ 141,430
Income tax at the US statutory federal rate (35%)	102,872	76,688	49,501
Increase (decrease) resulting from:
Foreign rate differential	(2,326)	1,434	5,047
State income taxes, net of federal benefit		66	(601)
Dividends paid deduction	(98,026)	(84,927)	(57,109)
Equity investments	3,293	4,297
Change in valuation allowance	(5,391)	(6,104)	6,174
Other items, net	2,259	1,716	(1,509)
Income tax expense (benefit)	$ 2,681	$ (6,830)	$ 1,503