Share-Based Arrangements (Restricted Stock Activity) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Nonqualified Stock Options and Restricted Stock
Restricted Stock, Weighted-Average Grant Date Fair Value [Roll Forward]
Tax benefit from compensation expense	$ 2,000,000	$ 2,000,000	$ 2,200,000
Compensation cost not yet recognized	$ 6,900,000
Compensation cost recognized, period	3 years
Employee Stock Options and ESPP Stock
Restricted Stock, Weighted-Average Grant Date Fair Value [Roll Forward]
Windfall tax benefit realized	$ 0	$ 0	300,000
Restricted Stock
Restricted Stock, Outstanding [Roll Forward]
Outstanding at beginning of period (in shares)	167,674
Granted (in shares)	79,508
Vested (in shares)	(78,571)
Forfeited or expired (in shares)	(13,729)
Outstanding at end of period (in shares)	154,882	167,674
Restricted Stock, Weighted-Average Grant Date Fair Value [Roll Forward]
Outstanding at beginning of period (in dollars per share)	$ 40.27
Granted (in dollars per share)	50.30
Vested (in dollars per share)	41.70
Forfeited or expired (in dollars per share)	42.46
Outstanding at end of period (in dollars per share)	$ 44.50	$ 40.27
Total fair value of restricted stock awards vested	$ 5,700,000	$ 4,300,000	$ 3,800,000