Common Stock Incentive Plan (Details) - 2012 Plan - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Plan disclosures
Plan term from the date of adoption	10 years
General and administrative expense
Plan disclosures
Share-based compensation costs recognized	$ 2.8	$ 2.6
Maximum
Plan disclosures
Authorized shares	3,985,734
Restricted Stock
Number of shares
Shares granted	114,118
Additional disclosures
Granted shares that vest over four year service period	72,626
Granted shares that vest over five year service period	41,492
Remaining unamortized share-based compensation expense	$ 23.8
Performance Shares
Additional disclosures
Awards vesting on the first and second January1 after the end of the three year performance period (as a percent)	50.00%
Performance Shares | Executive officers
Number of shares
Shares granted	111,637
Restricted Stock Units
Number of shares
Shares granted	10,191
Additional disclosures
Granted shares that vest over four year service period	6,161
Granted shares that vest over five year service period	4,030
Performance Shares and Restricted Stock Units
Additional disclosures
Remaining unamortized share-based compensation expense	$ 12.5