Common Stock Incentive Plan (Details) - 2012 Plan - USD ($) $ in Millions	1 Months Ended		3 Months Ended		9 Months Ended
	Jul. 31, 2018	May 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 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			$ 3.9	$ 3.4	$ 12.5	$ 10.6
Maximum
Common Stock Incentive Plan
Authorized shares			3,985,734		3,985,734
Restricted Stock
Common Stock Incentive Plan
Shares granted					147,000
Vesting period					4 years
Remaining unamortized share-based compensation expense			$ 16.7		$ 16.7
Restricted Stock | Directors
Common Stock Incentive Plan
Shares granted	8,000	28,000
Shares vested		20,000
Vesting period	3 years
Restricted Stock | Directors | Three year vesting period
Common Stock Incentive Plan
Shares granted		4,000
Vesting period		3 years
Restricted Stock | Directors | Two year vesting period
Common Stock Incentive Plan
Shares granted		4,000
Vesting period		2 years
Performance Shares and Restricted Stock Units
Common Stock Incentive Plan
Remaining unamortized share-based compensation expense			$ 13.6		$ 13.6
Performance Shares
Common Stock Incentive Plan
Vesting period					3 years
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%
Restricted Stock Units
Common Stock Incentive Plan
Shares granted					8,383
Vesting period					4 years