Long-term Debt - Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Credit Agreement [Member]
Debt Instrument [Line Items]
Debt	$ 0	$ 50,450
4.875% Senior Unsecured Notes Due 2025 [Member]
Debt Instrument [Line Items]
Debt	0	300,000
4.125% Senior Unsecured Notes Due 2029
Debt Instrument [Line Items]
Debt	400,000	0
Revolving Credit Facility [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Debt	0	50,450
Fair Value, Inputs, Level 2 [Member] | 4.875% Senior Unsecured Notes Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt, Fair Value	0	305,630
Fair Value, Inputs, Level 2 [Member] | 4.125% Senior Unsecured Notes Due 2029
Debt Instrument [Line Items]
Long-term Debt, Fair Value	408,500	0
Fair Value, Inputs, Level 2 [Member] | Revolving Credit Facility [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Long-term Debt, Fair Value	$ 0	$ 50,450