Share-Based Compensation and Benefit Plans (Employee Stock Purchase Plan Activity) (Details) - Employee stock purchase plan [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-Based Compensation and Benefit Plans
Compensation expense for shares issued under the ESPP	$ 2,212	$ 2,162	$ 2,065
Income tax benefit from compensation expense for shares issued under the ESPP	$ 844	$ 807	$ 773
Shares awarded or issued under employee benefit plans, shares	64	54	60
Weighted-average grant-date fair value of shares issued during the period in compensation and benefit plans other than stock options	$ 196.72	$ 227.12	$ 195.04