Debt - Narrative (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Debt Instrument [Line Items]
Amount of difference between fair value and book value	$ 35,500,000		$ 4,300,000
Weighted average interest rate on borrowings	3.50%	2.30%
Interest income from investments/other	$ 900,000	$ 900,000
Credit Agreements
Debt Instrument [Line Items]
Senior secured revolving credit facility amount	800,000,000
Allowable annual dividend payment under credit agreement	$ 60,000,000