Common Stock Incentive Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended			12 Months Ended			3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014 Incentive Award Plans	Dec. 31, 2013 Incentive Award Plans	Dec. 31, 2013 Incentive Award Plans Performance-based vesting	Mar. 31, 2014 Incentive Award Plans Performance-based vesting	Mar. 31, 2014 2012 Plan
Plan disclosures
Authorized shares							3,985,734
Plan term from the date of adoption							10 years
Share-based compensation costs recognized	$ 2.7	$ 3.8
Number of shares
Outstanding nonvested shares, beginning of year			722,263	895,550
Shares of common stock granted			223,293	484,060	51,454
Shares vested			(269,109)	(654,650)
Shares forfeited			(949)	(2,697)
Outstanding nonvested shares, end of each period			675,498	722,263
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)			$ 23.37	$ 19.94
Shares granted (in dollars per share)			$ 39.22	$ 41.13
Shares vested (in dollars per share)			$ 35.41	$ 30.91
Shares forfeited (in dollars per share)			$ 39.75	$ 37.30
Outstanding nonvested shares, end of each period (in dollars per share)			$ 33.90	$ 23.37
Additional disclosures
Granted shares that vest immediately			18,829
Shares of common stock granted			223,293	484,060	51,454
Granted shares that vest over four year service period			30,535
Granted shares that vest over five year service period			173,929
Remaining unamortized share-based compensation expense	$ 22.9
Granted shares vested in 2013 based on achievement of certain performance metrics						12,864
Granted shares which may vest at the end of 2014, if certain performance metrics are reached						12,864
Granted shares which may vest at the end of 2015, if certain performance metrics are reached						12,864
Granted shares which may vest at the end of 2016, if certain performance metrics are reached						12,864