Common Stock Incentive Plan (Details) - 2012 Plan - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	May 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 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			$ 4.5	$ 18.8
Maximum
Common Stock Incentive Plan
Authorized shares		3,985,734		3,985,734
Restricted Stock
Common Stock Incentive Plan
Share-based compensation costs recognized		$ 5.0		$ 8.7	$ 7.2
Shares granted	28,000			137,000
Shares vested	20,000
Restricted Stock | Two year vesting period
Common Stock Incentive Plan
Shares vested	4,000
Vesting period	2 years
Restricted Stock | Three year vesting period
Common Stock Incentive Plan
Shares vested	4,000
Vesting period	3 years
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		$ 15.8		$ 15.8
Performance Shares | Executive officers
Common Stock Incentive Plan
Shares granted				190,449
Performance Shares | Executive officers | Three year vesting period
Common Stock Incentive Plan
Vesting period				3 years
Awards vesting on the first and second January 1 after the end of the three year performance period (as a percent)				50.00%
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