Stockholders' Equity (Details 3) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Summary of restricted stock
Compensation expense related to long-term compensation plans, restricted stock and stock options capitalized to assets	$ 1,000,000	$ 1,000,000
Stock options
Stock based compensation
Options expiration period	10 years
Period of commencement of option vesting, from date of grant	1 year
Weighted average assumptions used for estimation of fair value of each stock option
Dividend yield (as a percent)	2.00%		2.00%
Expected life of option	4 years 8 months 12 days		3 years 8 months 12 days
Risk-free interest rate (as a percent)	0.68%		0.46%
Expected stock price volatility (as a percent)	35.50%		37.40%
Weighted average fair value of awards granted during the period	3,867,342		6,602,967
Options Outstanding
Balance at beginning of year (in shares)	1,201,000	1,277,200	1,277,200
Granted (in shares)	201,000		361,331
Exercised (in shares)	(79,036)		(382,612)
Lapsed or cancelled (in shares)	(4,000)		(54,919)
Balance at end of period (in shares)	1,318,964		1,201,000
Options exercisable at end of period (in shares)	585,786		479,913
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 75.05	$ 63.37	$ 63.37
Granted (in dollars per share)	$ 76.67		$ 75.36
Exercised (in dollars per share)	$ 50.22		$ 36.65
Lapsed or cancelled (in dollars per share)	$ 67.64		$ 72.99
Balance at end of period (in dollars per share)	$ 76.81		$ 75.05
Options exercisable at end of period (in dollars per share)	$ 81.38		$ 86.85
Exercise price of options granted, low end of the range (in dollars per share)	$ 20.67
Exercise price of options granted, high end of the range (in dollars per share)	$ 137.18
Remaining weighted average contractual life of the options outstanding	4 years 5 months 12 days
Remaining weighted average contractual life of the options exercisable	3 years 9 months
Share-based compensation, additional disclosures
Compensation expense	1,300,000	1,700,000
Total unrecognized compensation cost related to unvested stock awards	11,600,000
Weighted average period for recognition of compensation cost related to unvested stock awards	3 years
Stock options | Minimum
Stock based compensation
Options vesting period	1 year
Stock options | Maximum
Stock based compensation
Options vesting period	5 years
Restricted Stock Awards
Weighted average assumptions used for estimation of fair value of each stock option
Weighted average fair value of awards granted during the period			7,023,942
Share-based compensation, additional disclosures
Compensation expense	1,464,223		6,930,381
Total unrecognized compensation cost related to unvested stock awards	10,500,000
Weighted average period for recognition of compensation cost related to unvested stock awards	2 years 2 months 12 days
Annual award vesting rate, low end of range (as a percent)	15.00%
Annual award vesting rate, high end of range (as a percent)	35.00%
Summary of restricted stock
Balance at beginning of year (in shares)	2,804,901	2,912,456	2,912,456
Granted (in shares)			92,729
Cancelled (in shares)	(300)		(200,284)
Balance at end of period (in shares)	2,804,601		2,804,901
Vested during the period (in shares)	500		408,800
Fair value of restricted stock vested during the period	$ 20,000		$ 22,400,000