Stock-Based Compensation Plans (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock-Based Compensation Plans
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Shares
Exercised (in shares)	(1,902)
2009 Plan
Stock-Based Compensation Plans
Common Stock, Par or Stated Value Per Share	$ 0.01
Common stock shares available for grant	6,913,585
2009 Plan | Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	14,408	124,212	149,973
Exercised (in shares)	(1,902)	(109,804)	(32,756)
Granted (in shares)	0	0	6,995
Forfeited (in shares)	0	0	0
Options outstanding at the end of the period (in shares)	12,506	14,408	124,212
Options exercisable at the end of the period (in shares)	12,506	14,408	117,217
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share)	$ 124.05	$ 128.84	$ 126.77
Exercised (in dollars per share)	89.17	129.47	126.24
Granted (in dollars per share)	0	0	161.10
Forfeited (in dollars per share)	0	0	0
Options outstanding at the end of the period (in dollars per share)	129.35	124.05	128.84
Options exercisable at the end of the period (in dollars per share)	$ 129.35	$ 124.05	$ 126.91
1994 Plan | Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	0	0	7,778
Exercised (in shares)	0	0	(7,778)
Granted (in shares)	0	0	0
Forfeited (in shares)	0	0	0
Options outstanding at the end of the period (in shares)	0	0	0
Options exercisable at the end of the period (in shares)	0	0	0
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share)	$ 0	$ 0	$ 48.60
Exercised (in dollars per share)	0	0	48.60
Granted (in dollars per share)	0	0	0
Forfeited (in dollars per share)	0	0	0
Options outstanding at the end of the period (in dollars per share)	0	0	0
Options exercisable at the end of the period (in dollars per share)	$ 0	$ 0	$ 0