Common Stock Incentive Plan (Details) - 2012 Plan - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Common Stock Incentive Plan
Plan term from the date of adoption	10 years
General and administrative expense
Common Stock Incentive Plan
Share-based compensation costs recognized	$ 3.7	$ 2.8
Maximum
Common Stock Incentive Plan
Authorized shares	3,985,734
Restricted Stock
Common Stock Incentive Plan
Shares granted	100,553
Vesting period	4 years
Remaining unamortized share-based compensation expense	$ 20.8
Restricted Stock | Minimum
Common Stock Incentive Plan
Vesting period	4 years
Restricted Stock | Maximum
Common Stock Incentive Plan
Vesting period	5 years
Performance Shares and Restricted Stock Units
Common Stock Incentive Plan
Remaining unamortized share-based compensation expense	$ 17.7
Performance Shares | Executive officers
Common Stock Incentive Plan
Shares granted	190,449
Performance Shares | Executive officers | Three year vesting period
Common Stock Incentive Plan
Awards vesting on the first and second January 1 after the end of the three year performance period (as a percent)	50.00%
Vesting period	3 years
Restricted Stock Units
Common Stock Incentive Plan
Shares granted	8,383
Restricted Stock Units | Four year vesting period
Common Stock Incentive Plan
Vesting period	4 years