Significant Accounting Policies - Recently Adopted Accounting Standards (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2019	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	$ 2,540,203	$ 2,263,229
Other current assets	(48,324)	(37,211)
Deferred income taxes	(163,863)	(146,574)
Retained earnings	(2,841,546)	(2,615,131)
Operating lease, right-of-use asset	106,586	82,374
Present value of lease liabilities	$ 110,971	$ 85,163
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other current assets				$ 7,300
Deferred income taxes				6,700
Retained earnings				600
Operating lease, right-of-use asset			$ 68,000
Present value of lease liabilities			68,000
Finance lease, liability			68,000
Finance lease, right-of-use asset			$ 68,000
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				6,400
Cumulative Effect, Period of Adoption, Adjustment | AOCI Attributable to Parent
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				$ 6,400