Fair Value Measurements - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Jun. 27, 2012	Aug. 25, 2010
Fair Value, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, transfer in and out	$ 0	$ 0
Fair Value, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, transfer in and out	0	0
Fair Value, Level 2 | $250 Million Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instrument, fair value	257,000,000	269,200,000
Fair Value, Level 2 | $400 Million Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instrument, fair value	415,700,000	440,100,000
Fair Value, Level 2 | Senior Notes | $250 Million Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instrument, face amount				$ 250,000,000
Fair Value, Level 2 | Senior Notes | $400 Million Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instrument, face amount			$ 400,000,000
Fair Value, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value, transfer in and out	$ 0	$ 0