Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 41,537	$ 40,310
Average common shares outstanding - basic (in shares)	28,753	28,633
Additional shares for stock-based compensation plans (in shares)	50	90
Average common shares outstanding - diluted (in shares)	28,803	28,723
Earnings per share of common stock - basic (in dollars per share)	$ 1.44	$ 1.41
Earnings per share of common stock - diluted (in dollars per share)	$ 1.44	$ 1.40
Antidilutive shares (in shares)	16	22