Common Stock Incentive Plan (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Plan disclosures
Share-based compensation costs recognized	$ 2.8	$ 2.8	$ 5.4	$ 5.4
Additional disclosures
Remaining unamortized share-based compensation expense	$ 17.3		$ 17.3
Non-employee director
Number of shares
Shares granted			28,000
Additional disclosures
Shares granted			28,000
Granted shares that vest immediately	12,000		12,000
Granted Shares that Vest over one Year Service Period	8,000		8,000
Granted shares that vest over three year service period	8,000		8,000
Restricted Stock
Number of shares
Outstanding nonvested shares, beginning of year			527,176	722,263	722,263
Shares granted			153,785		262,655
Shares vested			(160,307)		(440,348)
Shares forfeited			(24,310)		(17,394)
Outstanding nonvested shares, end of each period	496,344		496,344		527,176
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)			$ 29.02	$ 23.37	$ 23.37
Shares granted (in dollars per share)			50.90		39.87
Shares vested (in dollars per share)			36.78		36.88
Shares forfeited (in dollars per share)			44.93		39.07
Outstanding nonvested shares, end of each period (in dollars per share)	$ 34.82		$ 34.82		$ 29.02
Additional disclosures
Shares granted			153,785		262,655
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		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		$ 46.94		$ 41.46
Additional disclosures
Shares granted			55,716		71,705
Remaining unamortized share-based compensation expense	$ 4.2		$ 4.2
Awards vesting (as a percent)			50.00%
Number of performance shares that vest based on the achievement of performance goals
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%
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		3,985,734