Common Stock Incentive Plan (Details) - 2012 Plan - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Plan disclosures
Plan term from the date of adoption			10 years
Share-based compensation costs recognized	$ 2.7	$ 2.2	$ 9.2	$ 7.6
Non-employee director
Number of shares
Shares granted			28,000
Additional disclosures
Granted shares that vest immediately			20,000
Granted shares that vest annually, in equal parts, over three year service period			8,000
Maximum
Plan disclosures
Authorized shares	3,985,734		3,985,734
Restricted Stock
Number of shares
Shares granted			170,539
Additional disclosures
Vesting period			5 years
Remaining unamortized share-based compensation expense	$ 16.0		$ 16.0
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			58,569
Restricted Stock Units
Number of shares
Shares granted			11,098
Additional disclosures
Vesting period			5 years
Retirement age			60 years
Performance Shares and Restricted Stock Units
Additional disclosures
Remaining unamortized share-based compensation expense	$ 4.9		$ 4.9