Share-Based Compensation and Benefit Plans (Summary of Restricted Stock) (Details) - Restricted stock [Member] - $ / shares shares in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-Based Compensation and Benefit Plans
Restricted stock granted during period, weighted-average grant date fair value		$ 253.78	$ 264.24	$ 208.56
Shares withheld to cover employees' taxes upon vesting		1
Employee [Member]
Share-Based Compensation and Benefit Plans
Non-vested restricted stock at beginning of the year, weighted-average grant date fair value		$ 204.33
Restricted stock granted during period, weighted-average grant date fair value		256.69
Restricted stock vested during the period, weighted-average grant date fair value	[1]	182.10
Restricted stock forfeited during the period, weighted-average grant date fair value		0.00
Non-vested restricted stock at the end of the year, weighted-average grant date fair value		$ 244.06	$ 204.33
Employee [Member] | Performance shares [Member]
Share-Based Compensation and Benefit Plans
Non-vested restricted stock beginning balance, shares		3
Restricted stock granted during the period, shares		1
Restricted stock vested during the period, shares	[1]	(1)
Restricted stock forfeited during the period, shares		0
Non-vested restricted stock ending balance, shares		3	3
Director [Member]
Share-Based Compensation and Benefit Plans
Non-vested restricted stock at beginning of the year, weighted-average grant date fair value		$ 222.77
Restricted stock granted during period, weighted-average grant date fair value		252.45
Restricted stock vested during the period, weighted-average grant date fair value		200.81
Restricted stock forfeited during the period, weighted-average grant date fair value		0.00
Non-vested restricted stock at the end of the year, weighted-average grant date fair value		$ 250.85	$ 222.77
Director [Member] | Performance shares [Member]
Share-Based Compensation and Benefit Plans
Non-vested restricted stock beginning balance, shares		6
Restricted stock granted during the period, shares		3
Restricted stock vested during the period, shares		(4)
Restricted stock forfeited during the period, shares		0
Non-vested restricted stock ending balance, shares		5	6

[1] Includes less than one thousand shares withheld to cover employees’ taxes upon vesting.