Common Stock Incentive Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended			12 Months Ended				9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Incentive Award Plans	Dec. 31, 2013 Incentive Award Plans	Dec. 31, 2013 Incentive Award Plans Performance-based vesting	Sep. 30, 2014 Incentive Award Plans Performance-based vesting	Sep. 30, 2014 Performance share awards	Sep. 30, 2014 Performance share awards Performance-based vesting	Sep. 30, 2014 Performance share awards Market-based vesting	Sep. 30, 2014 Performance share awards Executive officers	Sep. 30, 2014 2012 Plan
Plan disclosures
Authorized shares													3,985,734
Plan term from the date of adoption													10 years
Share-based compensation costs recognized	$ 2.2	$ 6.7	$ 7.7	$ 14.2
Number of shares
Outstanding nonvested shares, beginning of year					722,263	895,550
Shares granted					251,693	484,060	51,454					71,705
Shares vested					(321,408)	(654,650)
Shares forfeited					(17,394)	(2,697)
Outstanding nonvested shares, end of each period					635,154	722,263
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)					$ 23.37	$ 19.94
Shares granted (in dollars per share)					$ 39.68	$ 41.13						$ 41.46
Shares vested (in dollars per share)					$ 36.48	$ 30.91
Shares forfeited (in dollars per share)					$ 39.07	$ 37.30
Outstanding nonvested shares, end of each period (in dollars per share)					$ 29.83	$ 23.37
Additional disclosures
Granted shares that vest immediately					30,829
Granted shares that vest over two year service period					8,000
Granted shares that vest over three year service period					4,000
Shares granted					251,693	484,060	51,454					71,705
Granted shares that vest over four year service period					30,535
Granted shares that vest over five year service period					178,329
Remaining unamortized share-based compensation expense	18.9		18.9						2.6
Granted shares vested in 2013 based on achievement of certain performance metrics								12,864
Granted shares which may vest at the end of 2014, if certain performance metrics are reached								12,864
Granted shares which may vest at the end of 2015, if certain performance metrics are reached								12,864
Granted shares which may vest at the end of 2016, if certain performance metrics are reached								12,864
Total award value (as a percent)										20.00%	80.00%
Total award value relative to the MSCI US REIT Index (as a percent)											60.00%
Total award value relative to the NAREIT Freestanding Index (as a percent)											20.00%
Awards vesting on January 1, 2017 (as a percent)										50.00%
Awards vesting on January 1, 2018 (as a percent)										50.00%
Shares granted, estimated fair value												$ 3.0
Shares granted (in dollars per share)					$ 39.68	$ 41.13						$ 41.46