Common Stock Incentive Plan (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Plan disclosures
Share-based compensation costs recognized	$ 10.4	$ 12.0	$ 20.8
Additional disclosures
Remaining unamortized share-based compensation expense	$ 13.9
Vesting rights percentage	20.00%
Stock options outstanding	0	0	0
Total award value relative to the MSCI US REIT Index (as a percent)	50.00%	60.00%
TSR relative to NAREIT Freestanding Index (as a percent)	20.00%	20.00%
Dividend per share growth rate (as a percent)	20.00%
Debt-to-EBITDA ratio (as a percent)	10.00%	20.00%
Non-employee director
Number of shares
Shares granted	28,000
Additional disclosures
Shares granted	28,000
Granted shares that vest immediately	12,000
Granted Shares that Vest over one Year Service Period	8,000
Granted shares that vest over three year service period	8,000
Non-employee director | Less than six years of service
Additional disclosures
Years of service	6 years
Vesting rights percentage	33.33%
Non-employee director | Six years of service
Additional disclosures
Years of service	6 years
Vesting rights percentage	50.00%
Non-employee director | Seven years of service
Additional disclosures
Years of service	7 years
Vesting rights percentage	100.00%
Employee | Age 55 and below
Additional disclosures
Vesting rights percentage	20.00%
Employee | Age 56
Additional disclosures
Vesting rights percentage	25.00%
Eligible age of employees for vesting of awards	56 years
Employee | Age 57
Additional disclosures
Vesting rights percentage	33.33%
Eligible age of employees for vesting of awards	57 years
Employee | Age 58
Additional disclosures
Vesting rights percentage	50.00%
Eligible age of employees for vesting of awards	58 years
Employee | Age 59
Additional disclosures
Vesting rights percentage	100.00%
Eligible age of employees for vesting of awards	59 years
Employee | Age 60 and above
Additional disclosures
Eligible age of employees for vesting of awards	60 years
Non-executive employees
Additional disclosures
Years of service	6 months
Minimum | Non-employee director | Eight or more years of service
Additional disclosures
Years of service	8 years
Maximum | Employee | Age 55 and below
Additional disclosures
Eligible age of employees for vesting of awards	55 years
Restricted Stock
Number of shares
Outstanding nonvested shares, beginning of year	527,176	722,263	895,550
Shares granted	161,949	262,655	484,060
Shares vested	(205,248)	(440,348)	(654,650)
Shares forfeited	(27,595)	(17,394)	(2,697)
Outstanding nonvested shares, end of each period	456,282	527,176	722,263
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)	$ 29.02	$ 23.37	$ 19.94
Shares granted (in dollars per share)	50.87	39.87	41.13
Shares vested (in dollars per share)	37.70	36.88	30.91
Shares forfeited (in dollars per share)	45.58	39.07	37.30
Outstanding nonvested shares, end of each period (in dollars per share)	$ 30.46	$ 29.02	$ 23.37
Additional disclosures
Shares granted	161,949	262,655	484,060
Granted shares that vest immediately	4,964
Vesting period	5 years
Shares granted (in dollars per share)	$ 50.87	$ 39.87	$ 41.13
Restricted Stock | Non-executive employees
Additional disclosures
Number of shares of nonvested stock to be received after six full months of employment	200
Vesting period	5 years
Performance shares
Number of shares
Outstanding nonvested shares, beginning of year	59,405
Shares granted	55,716	71,705
Shares vested		(4,067)
Shares forfeited		(8,233)
Outstanding nonvested shares, end of each period	115,121	59,405
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)	$ 41.46
Shares granted (in dollars per share)	52.78	$ 41.46
Shares vested (in dollars per share)		41.46
Shares forfeited (in dollars per share)		41.46
Outstanding nonvested shares, end of each period (in dollars per share)	$ 46.94	$ 41.46
Additional disclosures
Shares granted	55,716	71,705
Remaining unamortized share-based compensation expense	$ 3.2
Awards vesting on the first and second January1 after the end of the three year performance period (as a percent)	50.00%
Shares granted (in dollars per share)	$ 52.78	$ 41.46
Restricted Stock Units
Number of shares
Shares granted	10,269
Additional disclosures
Shares granted	10,269
Vesting period	5 years
2012 Plan
Plan disclosures
Plan term from the date of adoption	10 years
2012 Plan | Maximum
Plan disclosures
Authorized shares	3,985,734