Stock Compensation Plans (Stock Purchase Plan) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock purchase plan
Compensation expense	$ 24,657,000	$ 22,102,000	$ 16,502,000
Stock Purchase Plan
Stock purchase plan
Purchase price of shares in terms compared to market price per share (as a percent)	90.00%
Annual purchase limitation per employee	$ 22,500
Compensation expense	$ 0
Common stock, shares authorized (in shares)	2,000,000
Shares sold to employees (in shares)	45,343	67,198
Value of shares sold to employees under purchase plan	$ 2,300,000	$ 2,100,000
Shares available (in shares)	526,687