Stock-Based Compensation Plans (Details 5) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Additional disclosures
Stock issued in connection with stock grants (in shares)		154,645
Stock-based compensation expense	$ 3,742	$ 3,883
Capitalized stock-based compensation cost	$ 3,048	$ 3,244
Performance Shares [Member]
Additional disclosures
Restricted stock granted	93,031
Restricted stock earned based on total shareholder return metrics related to common stock (in units)	60,229
Restricted stock earned based on operating performance and leverage metrics	32,802
Baseline share value (in dollars per share)	$ 161.56
Dividend yield (as a percent)	3.30%
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%
Average estimated fair value (in dollars per share)	$ 131.24
Outstanding unvested shares granted	251,276		238,266
Grant date fair value per share (in dollars per share)	$ 141.88
Restricted stock
Additional disclosures
Restricted stock granted	77,553	58,819
Share Based Compensation Arrangement by Share Based Payment Award Equity Instruments Other than Options Issued in Period Fair Value	$ 12,529	$ 10,199
Outstanding unvested shares granted	145,530		147,884
Grant date fair value per share (in dollars per share)	$ 161.56
Restricted stock and restricted stock units
Additional disclosures
Unrecognized compensation cost for unvested restricted stock	$ 40,083
Weighted average period for recognition of unrecognized compensation cost	3 years 10 months 24 days