Common Stock Incentive Plan (Details) - 2012 Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Plan disclosures
Plan term from the date of adoption			10 years
General and administrative expense
Plan disclosures
Share-based compensation costs recognized	$ 3.4	$ 2.7	$ 10.6	$ 9.2
Maximum
Plan disclosures
Authorized shares	3,985,734		3,985,734
Restricted Stock
Plan disclosures
Shares granted			119,564
Remaining unamortized share-based compensation expense	$ 20.2		$ 20.2
Restricted Stock | Four year service period
Plan disclosures
Shares granted			72,626
Vesting period			4 years
Restricted Stock | Five year service period
Plan disclosures
Shares granted			46,938
Vesting period			5 years
Restricted Stock | Independent directors
Plan disclosures
Shares granted			28,000
Restricted Stock | Independent directors | Immediately vested
Plan disclosures
Shares granted			20,000
Restricted Stock | Independent directors | Three year service period
Plan disclosures
Shares granted			8,000
Vesting period			3 years
Performance Shares and Restricted Stock Units
Plan disclosures
Remaining unamortized share-based compensation expense	$ 10.0		$ 10.0
Performance Shares | Executive officers
Plan disclosures
Shares granted			111,637
Performance Shares | Executive officers | Three year service period
Plan disclosures
Vesting period			3 years
Awards vesting on the first and second January1 after the end of the three year performance period (as a percent)			50.00%
Restricted Stock Units
Plan disclosures
Shares granted			10,191
Restricted Stock Units | Four year service period
Plan disclosures
Shares granted			6,161
Vesting period			4 years
Restricted Stock Units | Five year service period
Plan disclosures
Shares granted			4,030
Vesting period			5 years