Stock-Based Compensation Plans (Details 5) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Additional disclosures
Stock-based compensation expense	$ 8,045	$ 7,777
Capitalized stock-based compensation cost	$ 6,052	$ 6,071
Performance Shares [Member]
Additional disclosures
Restricted stock granted	93,525
Dividend yield (as a percent)	3.30%
Average estimated fair value (in dollars per share)	$ 131.24
Estimated volatility, Minimum (as a percent)	15.20%
Estimated volatility, Maximum (as a percent)	22.80%
Historical volatility (as a percent)	50.00%
Implied volatility (as a percent)	50.00%
Risk-free interest rate, minimum (as a percent)	0.44%
Risk-free interest rate, maximum (as a percent)	0.88%
Baseline share value (in dollars per share)	$ 161.56
Outstanding unvested shares granted	251,140		238,266
Grant date fair value per share (in dollars per share)	$ 141.88
Restricted stock
Additional disclosures
Restricted stock granted	80,441	61,953
Outstanding unvested shares granted	142,745		147,884
Grant date fair value per share (in dollars per share)	$ 162.22
Restricted stock and restricted stock units
Additional disclosures
Unrecognized compensation cost for unvested restricted stock	$ 33,436
Weighted average period for recognition of unrecognized compensation cost	3 years 9 months 18 days