Equity Incentive Plan (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended
	Feb. 28, 2018	Feb. 28, 2017	Mar. 31, 2018	Mar. 31, 2017
Equity Incentive Plan
Maximum number of common shares available for issuance (in shares)			7,500,000
Other Disclosures
Common shares available for future grant (in shares)			3,102,567
Restricted share awards
Summary of non-vested shares/units
Unvested at the beginning of the period (in shares)			700,325
Granted (in shares)			360,416
Vested (in shares)			(65,341)
Forfeited (in shares)			(2,479)
Unvested at the end of the period (in shares)			992,921
Weighted Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share)			$ 22.88
Granted (in dollars per share)			21.10
Vested (in dollars per share)			24.10
Forfeited (in dollars per share)			22.84
Unvested at the end of the period (in dollars per share)			$ 22.15
Other Disclosures
Share-based compensation expense			$ 2.0	$ 2.0
Total unrecognized compensation costs			$ 19.6
Weighted-average period of recognition of unrecognized share-based compensation expense			2 years 9 months
Total fair value of shares vested			$ 1.4	1.4
2017 Performance Shares
Summary of non-vested shares/units
Granted (in shares)		259,000
Weighted Average Grant Date Fair Value
Granted (in dollars per share)		$ 14.93
Other Disclosures
Vesting period		4 years
Performance-based vesting period		3 years
Time-based vesting period		1 year
2018 Performance Shares [Member]
Summary of non-vested shares/units
Granted (in shares)	264,000
Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 13.99
Other Disclosures
Vesting period	4 years
Performance-based vesting period	3 years
Time-based vesting period	1 year
Vesting percentage upon satisfaction of performance-based vesting period	50.00%
Vesting percentage upon satisfaction of time-based vesting period	50.00%
Fair value assumptions, risk free interest rate	2.42%
Fair value assumptions, expected volatility rate	27.44%
Percentage of grant date fair value to be recognized over three years	50.00%
Employee service share based compensation cost period of recognition	3 years
Percentage of grant date fair value to be recognized over four years	50.00%
Employee service share based compensation cost period of recognition	4 years
Performance Units
Other Disclosures
Share-based compensation expense			0.5	$ 0.3
Total unrecognized compensation costs			$ 7.5
Weighted-average period of recognition of unrecognized share-based compensation expense			2 years 9 months
Minimum | 2018 Performance Shares [Member]
Other Disclosures
Percentage of performance units that will convert into restricted shares	25.00%
Maximum | 2018 Performance Shares [Member]
Other Disclosures
Percentage of performance units that will convert into restricted shares	150.00%