Debt - Credit Facility (Details) - Line of Credit		12 Months Ended
	Jan. 07, 2015 USD ($)	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Credit facility term	5 years
Potential increase in revolving credit facility commitment	$ 250,000,000
Amount outstanding		$ 0
Consolidated leverage ratio (not more than)	3.5
Consolidated interest coverage ratio (not less than)	3.0
Letter of Credit
Line of Credit Facility [Line Items]
Unsecured credit facility maximum amount	$ 25,000,000
Swing Line Loans
Line of Credit Facility [Line Items]
Unsecured credit facility maximum amount	$ 10,000,000
LIBOR
Line of Credit Facility [Line Items]
Basis spread on LIBOR (percent)		1.10%
Minimum
Line of Credit Facility [Line Items]
Quarterly facility fee (percent)		0.125%
Minimum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on LIBOR (percent)		1.00%
Maximum
Line of Credit Facility [Line Items]
Quarterly facility fee (percent)		0.20%
Maximum | LIBOR
Line of Credit Facility [Line Items]
Basis spread on LIBOR (percent)		1.30%