Fair Value Measurements - Carrying Amounts and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Measurements [Line Items]
Long-term debt	$ 1,060,037	$ 1,058,738
Senior | $450 Million Senior Unsecured Notes Due 2031 | Level 2 | Carrying Amount
Fair Value Measurements [Line Items]
Long-term debt	444,318	443,860
Senior | $450 Million Senior Unsecured Notes Due 2031 | Level 2 | Fair Value
Fair Value Measurements [Line Items]
Long-term debt	484,574	498,290
Senior | $400 Million Senior Unsecured Notes Due 2029 | Level 2 | Carrying Amount
Fair Value Measurements [Line Items]
Long-term debt	395,086	394,635
Senior | $400 Million Senior Unsecured Notes Due 2029 | Level 2 | Fair Value
Fair Value Measurements [Line Items]
Long-term debt	428,544	438,104
Senior | $400 Million Senior Notes Due 2022 | Level 2 | Carrying Amount
Fair Value Measurements [Line Items]
Long-term debt	216,217	215,827
Senior | $400 Million Senior Notes Due 2022 | Level 2 | Fair Value
Fair Value Measurements [Line Items]
Long-term debt	$ 224,688	$ 232,381