Credit Facility (Details) - Line of Credit [Member]		9 Months Ended
	Jan. 07, 2015 USD ($)	Sep. 30, 2017 USD ($)
Line of Credit Facility [Line Items]
Term of credit facility	5 years
Aggregate amount	$ 250,000,000
Additional borrowing capacity	$ 250,000,000
Weighted average interest rate on amounts outstanding (percent)		1.57%
Amount outstanding		$ 40,000,000
Consolidated leverage ratio	3.5
Consolidated interest coverage ratio	3.0
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Aggregate amount	$ 25,000,000
Swing Line Loans [Member]
Line of Credit Facility [Line Items]
Aggregate amount	$ 10,000,000
LIBOR [Member]
Line of Credit Facility [Line Items]
Basis spread on LIBOR (percent)		1.10%
Minimum [Member]
Line of Credit Facility [Line Items]
Quarterly facility fee (percent)		0.125%
Minimum [Member] | LIBOR [Member]
Line of Credit Facility [Line Items]
Basis spread on LIBOR (percent)		1.00%
Maximum [Member]
Line of Credit Facility [Line Items]
Quarterly facility fee (percent)		0.20%
Maximum [Member] | LIBOR [Member]
Line of Credit Facility [Line Items]
Basis spread on LIBOR (percent)		1.30%