Stock-Based Compensation Plans (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-Based Compensation Plans
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Shares
Exercised (in shares)	(40,534)
2009 Plan
Stock-Based Compensation Plans
Common stock, par value (in dollars per share)	$ 0.01
Common stock shares available for grant	7,509,205
2009 Plan | Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	149,973	177,333	249,178
Exercised (in shares)	(32,756)	(27,360)	(71,845)
Granted (in shares)	6,995	0	0
Forfeited (in shares)	0	0	0
Options outstanding at the end of the period (in shares)	124,212	149,973	177,333
Options exercisable at the end of the period (in shares)	117,217	149,973	177,333
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share)	$ 126.77	$ 124.25	$ 122.17
Exercised (in dollars per share)	126.24	110.47	117.04
Granted (in dollars per share)	161.10	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)	128.84	126.77	124.25
Options exercisable at the end of the period (in dollars per share)	$ 126.91	$ 126.77	$ 124.25
1994 Plan | Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	7,778	22,541	82,195
Exercised (in shares)	(7,778)	(14,763)	(59,654)
Granted (in shares)	0	0	0
Forfeited (in shares)	0	0	0
Options outstanding at the end of the period (in shares)	0	7,778	22,541
Options exercisable at the end of the period (in shares)	0	7,778	22,541
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share)	$ 48.60	$ 77.91	$ 103.27
Exercised (in dollars per share)	48.60	93.35	112.85
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)	0.00	48.60	77.91
Options exercisable at the end of the period (in dollars per share)	$ 0.00	$ 48.60	$ 77.91