Employee Stock and Incentive Plans - Performance Shares (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011 Years	Dec. 31, 2011 Performance Shares Awards [Member]		Dec. 31, 2010 Performance Shares Awards [Member]		Dec. 31, 2009 Performance Shares Awards [Member]
Number of Shares [Abstract]
Non-vested as of January 1,				122,489		121,825		79,238
Granted				39,312	[1]	49,302	[1]	69,800	[1]
Awarded		100,000		(32,368)		0		0
Unearned Grant Award				0		(22,909)		(24,615)
Forfeited				(1,100)		(25,729)		(2,598)
Non-vested as of December 31,				128,333		122,489		121,825
Weighted Average Grant Date Fair Value [Abstract]
As of January 1,				$ 38.15		$ 41.96		$ 47.94
Granted				$ 41.00	[1]	$ 35.44	[1]	$ 35.06	[1]
Awarded				$ 48.10		$ 0.00		$ 0.00
Unearned Grant Award				$ 0.00		$ 54.50		$ 41.97
Forfeited				$ 34.35		$ 36.45		$ 38.78
As of December 31,				$ 28.00		$ 38.15		$ 41.96
Performance Share Grants, Grant Date Fair Value			$ 1.4
Performance Share Awards, Grant Date Fair Value	$ 1.4		$ 1.6
Performance Period			3

[1] (a) Shares granted includes accrued dividends.