Stockholders' Equity (Narrative) (Details) - USD ($)	1 Months Ended		3 Months Ended	6 Months Ended	9 Months Ended
	Nov. 09, 2015	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2015	Jan. 30, 2015
2015 Stock Incentive Plan
Stockholders' equity
Common stock reserved for issuance (in shares)		1,275,000	1,275,000	1,275,000	1,275,000
Maximum number of shares that may be issued through the exercise of incentive stock options		750,000	750,000	750,000	750,000
Compensation cost not yet recognized		$ 13,100,000	$ 13,100,000	$ 13,100,000	$ 13,100,000
2015 Stock Incentive Plan | Time-based equity award
Stockholders' equity
Number of shares granted			12,000	12,000
Weighted average grant date fair value			$ 63.66	$ 63.66
Award vesting period of the grants				3 years
2015 Employee Stock Purchase Plan
Stockholders' equity
Common stock reserved for issuance (in shares)		1,000,000	1,000,000	1,000,000	1,000,000
Employee stock purchase plan, purchase period		6 months
Employee stock purchase plan discount		15.00%
2006 Equity Incentive Plans | Nonvested stock units issued (NSUs)
Stockholders' equity
Number of shares granted			2,000	185,000
Weighted average grant date fair value			$ 73.34	$ 74.22
NSUs granted that will vest at the end of each of the three years after the performance goals are achieved (as a percent)				33.33%
2006 Equity Incentive Plans | Time-based equity award
Stockholders' equity
Number of shares granted			3,000	8,000
Weighted average grant date fair value			$ 72.25	$ 73.43
2015 Stock Repurchase Program
Stockholders' equity
Maximum stock repurchase amount approved by Board of Directors						$ 200,000,000
Treasury stock, shares, acquired				979,000
Treasury stock, value, acquired				$ 69,201,000
Treasury stock acquired, average cost per share				$ 70.69	$ 71.64
Treasury stock, number of shares held		1,356,000	1,356,000	1,356,000	1,356,000
Treasury stock, value		$ 97,100,000	$ 97,100,000	$ 97,100,000	$ 97,100,000
Remaining stock repurchase amount approved by Board of Directors		$ 102,900,000	$ 102,900,000	$ 102,900,000	$ 102,900,000
Subsequent Event | 2015 Stock Incentive Plan | Time-based equity award
Stockholders' equity
Number of shares granted	5,000
Weighted average grant date fair value	$ 55.21