Common Stock Incentive Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended			0 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	1 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 John Case Performance-based vesting item	Jul. 02, 2013 Incentive Award Plans	Sep. 30, 2013 Incentive Award Plans	Sep. 30, 2013 Incentive Award Plans	Dec. 31, 2012 Incentive Award Plans	Sep. 30, 2013 Incentive Award Plans Performance-based vesting	Sep. 30, 2013 Incentive Award Plans John Case	Sep. 30, 2013 Incentive Award Plans John Case Performance-based vesting	Sep. 30, 2013 Incentive Award Plans Maximum	Sep. 30, 2013 2012 Plan
Plan disclosures
Authorized shares														3,985,734
Plan term from the date of adoption														10 years
Share-based compensation costs recognized	$ 6.7	$ 2.2	$ 14.2	$ 7.8
Vesting period						5 years		10 years			4 years		5 years
Number of shares
Outstanding nonvested shares, beginning of year								895,550	925,526
Shares of common stock granted								402,296	261,811	51,454	77,180	51,454
Shares vested						212,827		(541,774)	(290,877)
Shares forfeited								(2,646)	(910)
Outstanding nonvested shares, end of each period							753,426	753,426	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)								$ 41.89	$ 35.06
Shares vested (in dollars per share)								$ 29.43	$ 27.47
Shares forfeited (in dollars per share)								$ 37.35	$ 31.67
Outstanding nonvested shares, end of each period (in dollars per share)							$ 27.04	$ 27.04	$ 19.94
Additional disclosures
Granted shares that vest immediately							25,662	25,662
Granted Shares that Vest over one Year Service Period							62,989	62,989
Granted shares that vest over three year service period							12,000	12,000
Granted shares that vest over four year service period							77,180	77,180
Granted shares that vest over five year service period							173,011	173,011
Additional compensation expense							3.7	3.7
Granted shares that vest at the end of 2013 if certain performance metrics are reached										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
Remaining unamortized share-based compensation expense							$ 20.4	$ 20.4
Aggregate number of shares that can be vested												12,864
Number of annual performance metrics to be achieved for vesting shares					7