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Fair Value Measurements - Narrative (Details) - Senior Notes [Member] - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Oct. 31, 2019
Sep. 30, 2019
Apr. 15, 2015
$400 Million 5% Senior Notes [Member]          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Face amount $ 400,000,000       $ 400,000,000.0
Stated interest rate (as a percent) 5.00%       5.00%
Debt amount $ 397,699,000 $ 396,778,000      
Fair value of notes 403,000,000        
$700 Million 4.75% Senior Note [Member]          
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]          
Face amount $ 700,000,000   $ 700,000,000 $ 700,000,000  
Stated interest rate (as a percent) 4.75%   4.75% 4.75%  
Debt amount $ 691,491,000 $ 690,626,000      
Fair value of notes $ 726,000,000