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, 2013	Mar. 31, 2012	Mar. 31, 2013 Incentive Award Plans	Dec. 31, 2012 Incentive Award Plans	Mar. 31, 2013 2012 Plan	Mar. 31, 2013 2012 Plan Maximum
Plan disclosures
Authorized shares					3,985,734
Plan term from the date of adoption					10 years
Vesting period						10 years
Share-based compensation costs recognized	$ 3.8	$ 3.0
Number of shares
Outstanding nonvested shares, beginning of year			895,550	925,526
Shares granted			246,987	261,811
Shares vested			(287,328)	(290,877)
Shares forfeited			(1,291)	(910)
Outstanding nonvested shares, end of each period			853,918	895,550
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)			$ 19.94	$ 20.21
Shares granted (in dollars per share)			$ 41.98	$ 35.06
Shares vested (in dollars per share)			$ 31.31	$ 27.47
Shares forfeited (in dollars per share)			$ 31.31	$ 31.67
Outstanding nonvested shares, end of each period (in dollars per share)			$ 28.50	$ 19.94
Additional disclosures
Granted shares that vest immediately			17,662
Granted shares that vest over two year service period			58,989
Granted shares that vest over five year service period			170,336
Remaining unamortized share-based compensation expense	$ 24.3