Earnings Per Share (Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net income	$ 80,244	$ 8,175	$ 76,388	$ 98,582	$ 91,028	$ 88,132	$ 164,807	$ 277,742
Weighted-average number of common shares outstanding (for basic calculation)	27,973			27,775			27,892	27,730
Dilutive stock awards	481			613			478	538
Weighted-average number of common shares outstanding (for diluted calculation)	28,454			28,388			28,370	28,268
Basic earnings per share	$ 2.87			$ 3.55			$ 5.91	$ 10.02
Diluted earnings per share	$ 2.82			$ 3.47			$ 5.81	$ 9.83