Debt - Additional Information (Detail) - USD ($)		12 Months Ended
	Jan. 20, 2016	Dec. 31, 2017
Debt Instrument [Line Items]
Term of loan		5 years
Agreement date		Apr. 18, 2014
Aggregate loan amount		$ 750,000,000
Sub-facility for letter of credit		150,000,000
Sub-facility for swing line loans		50,000,000
Increase in aggregate principal amount		$ 250,000,000
Base rate	0.75%
Maximum capitalization ratio	45.00%	50.00%
Minimum interest coverage ratio		300.00%
Minimum asset coverage ratio	150.00%
Percentage increase in commitment fee	0.05%
Percentage of equity in customary security interest	65.00%
Senior secured revolving credit facility commitment		$ 750,000,000
Line of Credit Facility, Expiration Date		Apr. 18, 2019
Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility borrowings	$ 40,000,000
Swing Line Loan [Member]
Debt Instrument [Line Items]
Line of credit facility borrowings	$ 25,000,000
Accordion Debt [member]
Debt Instrument [Line Items]
Senior secured revolving credit facility commitment		$ 250,000,000
Senior Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility borrowings		162,000,000
Line of Credit Facility, Available Borrowing Capacity		$ 429,000,000
Line Of credit Unused Capacity Percentage		72.00%
Letters of credit		$ 6,000,000
Casualty insurance, expiration month and year		2018-07
Minimum [Member]
Debt Instrument [Line Items]
Annual commitment fee range		0.25%
Line of credit covenant trigger percentage		25.00%
Maximum [Member]
Debt Instrument [Line Items]
Annual commitment fee range		0.35%
Federal Funds Rate [Member]
Debt Instrument [Line Items]
Base rate		0.50%
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Base rate		1.00%