Compensation and Benefit Plans (Performance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Performance Shares
Performance Shares, Weighted-Average Grant-Date Fair Value
Vesting, market condition, performance period (in years)	3 years
Unrecognized compensation cost	$ 4.0
Weighted-average period, recognition of unrecognized compensation cost (in years)	2 years
Performance Shares, Equity Awards
Performance Shares
Nonvested, at the beginning of the period (in shares)	84,313
Granted (in shares)	16,797
Forfeited (in shares)	(1,514)	[1]
Vested (shares)	(52,650)	[1]
Transferred to affiliates (in shares)	(5,717)
Nonvested, at the end of the period (in shares)	41,229		84,313
Performance Shares, Weighted-Average Grant-Date Fair Value
Nonvested, at the beginning of the period (in dollars per share)	$ 27.50
Granted (in dollars per share)	$ 51.41	[2]	$ 31.14	[2]	$ 32.50	[2]
Forfeited (in dollars per share)	$ 37.56	[1]
Vested (in dollars per share)	$ 25.71	[1]
Transferred to affiliates (in dollars per share)	$ 27.62
Nonvested, at the end of the period (in dollars per share)	$ 39.14		$ 27.50
Performance Shares, Liability Awards
Performance Shares
Nonvested, at the beginning of the period (in shares)	84,313
Granted (in shares)	16,750
Forfeited (in shares)	(1,508)	[1]
Vested (shares)	(52,678)	[1]
Transferred to affiliates (in shares)	(5,716)
Nonvested, at the end of the period (in shares)	41,161
Performance Shares, Weighted-Average Grant-Date Fair Value
Nonvested, at the beginning of the period (in dollars per share)	$ 29.48
Nonvested, at the end of the period (in dollars per share)	$ 46.48

[1] Includes performance shares that were paid as performance targets were met.

[2] There were no settlements of awards for performance shares in 2012, 2011 and 2010 as performance targets were not met.