Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share
Antidilutive common share equivalents excluded from computation of earnings per share amount (in shares)	0	0	0	0
Net Income (Numerator)
Basic earnings per share (in dollars)	$ 194,302	$ 362,449	$ 398,630	$ 590,000
Diluted earnings per share (in dollars)	$ 194,302	$ 362,449	$ 398,630	$ 590,000
Weighted Average Shares (Denominator)
Basic earnings per share (in shares)	221,505,000	235,617,000	222,781,000	236,120,000
Dilutive common share equivalents (in shares)	1,014,000	1,328,000	960,000	1,214,000
Diluted earnings per share (in shares)	222,519,000	236,945,000	223,741,000	237,334,000
Per Share Amount
Basic (in dollars per share)	$ 0.88	$ 1.54	$ 1.79	$ 2.50
Diluted (in dollars per share)	$ 0.87	$ 1.53	$ 1.78	$ 2.49