Common Stock Incentive Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Plan disclosures
Share-based compensation costs recognized	$ 2.6	$ 2.7
Additional disclosures
Remaining unamortized share-based compensation expense	18.4
Restricted Stock
Number of shares
Outstanding nonvested shares, beginning of year	527,176	722,263	722,263
Shares granted	123,585		262,655
Shares vested	(118,630)		(440,348)
Shares forfeited	(21,810)		(17,394)
Outstanding nonvested shares, end of each period	510,321		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)	$ 51.86		$ 39.87
Shares vested (in dollars per share)	$ 35.20		$ 36.88
Shares forfeited (in dollars per share)	$ 45.13		$ 39.07
Outstanding nonvested shares, end of each period (in dollars per share)	$ 36.07		$ 29.02
Additional disclosures
Shares granted	123,585		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		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	$ 4.6
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
Authorized shares	3,985,734
Plan term from the date of adoption	10 years