Stock-Based Compensation Plans (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-Based Compensation Plans
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Shares
Exercised (in shares)	(42,123)
2009 Plan
Stock-Based Compensation Plans
Common stock, par value (in dollars per share)	$ 0.01
Common stock shares available for grant	7,994,292
2009 Plan | Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	177,333	249,178	340,062
Exercised (in shares)	(27,360)	(71,845)	(90,884)
Granted (in shares)	0	0	0
Forfeited (in shares)	0	0	0
Options outstanding at the end of the period (in shares)	149,973	177,333	249,178
Options exercisable at the end of the period (in shares)	149,973	177,333	188,081
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share)	$ 124.25	$ 122.17	$ 122.67
Exercised (in dollars per share)	110.47	117.04	124.01
Granted (in dollars per share)	0.00	0.00	0.00
Forfeited (in dollars per share)	0.00	0.00	0.00
Options outstanding at the end of the period (in dollars per share)	126.77	124.25	122.17
Options exercisable at the end of the period (in dollars per share)	$ 126.77	$ 124.25	$ 119.98
1994 Plan | Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	22,541	82,195	272,402
Exercised (in shares)	(14,763)	(59,654)	(190,207)
Granted (in shares)	0	0	0
Forfeited (in shares)	0	0	0
Options outstanding at the end of the period (in shares)	7,778	22,541	82,195
Options exercisable at the end of the period (in shares)	7,778	22,541	82,195
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share)	$ 77.91	$ 103.27	$ 104.96
Exercised (in dollars per share)	93.35	112.85	105.70
Granted (in dollars per share)	0.00	0.00	0.00
Forfeited (in dollars per share)	0.00	0.00	0.00
Options outstanding at the end of the period (in dollars per share)	48.60	77.91	103.27
Options exercisable at the end of the period (in dollars per share)	$ 48.60	$ 77.91	$ 103.27