Fair Value Measurements Fair Value Measurements, Financial Instruments Measured On a Recurring Basis, Quantitative Information (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended				12 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Dec. 31, 2011
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investment Grade Tax Exempt Securities Yield Spread Over Comparable Securities Minimum	75		75
Investment Grade Tax Exempt Securities Yield Spread Over Comparable Securities Maximum	80		80
Below Investment Grade Tax Exempt Securities Yield Spread Over Comparable Securities	700		600
Average Yields On Comparable Short-term Taxable Securities Maximum	1.00%		1.00%
Fair Value, Inputs, Level 3 [Member] | Municipal and Other Tax-exempt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Asset, Par Value	$ 46,100		$ 46,750		$ 46,200
Fair Value Inputs, Asset, Amortized Cost	42,395	[1]	43,706	[1]	42,492	[1]
Fair Value Inputs, Asset, Fair Value	41,279		43,712		42,353
Fair Value, Inputs, Level 3 [Member] | Municipal and Other Tax-exempt Securities [Member] | External Credit Rating, Investment Grade [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Asset, Par Value	29,100		29,750		29,200
Fair Value Inputs, Asset, Amortized Cost	28,999	[1]	29,643	[1]	29,466	[1]
Fair Value Inputs, Asset, Fair Value	28,848		29,532		29,327
Fair Value Measurements, Valuation Techniques	Discounted cash flows	[2]	Discounted cash flows1	[3]	Discounted cash flows1	[2]
Fair Value Measurements, Significant Unobservable Input	Interest rate spread		Interest rate spread		Interest rate spread
Fair Value Inputs, Assets, Discount Rate, Minimum	1.00%	[4]	1.00%	[4]	1.00%	[4]
Fair Value Inputs, Assets, Discount Rate, Maximum	1.50%	[4]	1.30%	[4]	1.50%	[4]
Fair Value Inputs, Assets, Discount Rate, Weighted Average	1.25%	[4]	1.21%	[4]	1.33%	[4]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value, Minimum	98.85%	[5]	98.99%	[5]	98.79%	[5]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value, Maximum	99.47%	[5]	98.48%	[5]	99.60%	[5]
Fair Value Inputs, Fair Value As Percentage of Par Value, Weighted Average	99.13%	[5]	99.35%	[5]	99.24%	[5]
Additional Decrease in Fair Value From a 100 Basis Point Increase in Interest Rate Spread	(285)
Fair Value, Inputs, Level 3 [Member] | Municipal and Other Tax-exempt Securities [Member] | External Credit Rating, Non Investment Grade [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Asset, Par Value	17,000		17,000		17,000
Fair Value Inputs, Asset, Amortized Cost	13,396	[1]	14,063	[1]	13,026	[1]
Fair Value Inputs, Asset, Fair Value	12,431		14,180		13,026
Fair Value Measurements, Valuation Techniques	Discounted cash flows	[2]	Discounted cash flows1	[3]	Discounted cash flows1	[2]
Fair Value Measurements, Significant Unobservable Input	Interest rate spread		Interest rate spread		Interest rate spread
Fair Value Inputs, Assets, Discount Rate, Minimum	7.20%	[6]	6.25%	[7]	6.25%	[7]
Fair Value Inputs, Assets, Discount Rate, Maximum	9.88%	[6]	9.55%	[7]	9.58%	[7]
Fair Value Inputs, Assets, Discount Rate, Weighted Average	7.77%	[6]	6.66%	[7]	6.93%	[7]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value, Minimum	73.06%	[5]	83.35%	[8]	76.45%	[5]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value, Maximum	73.30%	[5]	83.57%	[8]	76.99%	[5]
Fair Value Inputs, Fair Value As Percentage of Par Value, Weighted Average	73.13%	[5]	83.41%	[8]	76.62%	[5]
Additional Decrease in Fair Value From a 100 Basis Point Increase in Interest Rate Spread	(363)
Fair Value, Inputs, Level 3 [Member] | Other Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Asset, Par Value	5,400		5,900		5,900
Fair Value Inputs, Asset, Amortized Cost	5,400	[1]	5,900	[1]	5,900	[1]
Fair Value Inputs, Asset, Fair Value	5,400		5,900		5,900
Fair Value Measurements, Valuation Techniques	Discounted cash flows	[2]	Discounted cash flows1	[3]	Discounted cash flows1	[2]
Fair Value Measurements, Significant Unobservable Input	Interest rate spread		Interest rate spread		Interest rate spread
Fair Value Inputs, Assets, Discount Rate, Minimum	1.70%	[9]	1.60%	[10]	1.60%	[9]
Fair Value Inputs, Assets, Discount Rate, Maximum	1.73%	[9]	1.73%	[10]	1.80%	[9]
Fair Value Inputs, Assets, Discount Rate, Weighted Average	1.71%	[9]	1.70%	[10]	1.76%	[9]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value, Minimum	100.00%	[5]	100.00%	[5]	100.00%	[5]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value, Maximum	100.00%	[5]	100.00%	[5]	100.00%	[5]
Fair Value Inputs, Fair Value As Percentage of Par Value, Weighted Average	100.00%	[5]	100.00%	[5]	100.00%	[5]
Additional Decrease in Fair Value From a 100 Basis Point Increase in Interest Rate Spread	(53)
Fair Value, Inputs, Level 3 [Member] | Other assets - private equity funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Asset, Fair Value	$ 28,792		$ 33,415		$ 30,902
Fair Value Measurements, Valuation Techniques	Net asset value reported by underlying fund		Net asset value reported by underlying fund		Net asset value reported by underlying fund
Fair Value Measurements, Significant Unobservable Input	Net asset value reported by underlying fund		Net asset value reported by underlying fund		Net asset value reported by underlying fund

[1] Amortized cost reduced by other-than-temporary impairments recorded in earnings. See Note 2 for additional discussion.

[2] Discounted cash flows developed using discount rates primarily based on reference to interest rate spreads for comparable securities of similar duration and credit rating as determined by the nationally-recognized rating agencies, adjusted for lack of trading volume

[3] 1 Discounted cash flows developed using discount rates primarily based on reference to interest rate spreads for comparable securities of similar duration and credit rating as determined by the nationally-recognized rating agencies, adjusted for lack of trading volume

[4] Interest rate yields used to value investment grade tax-exempt securities represent a spread of 75 to 80 basis points over average yields for comparable tax-exempt securities.

[5] Represents fair value as a percentage of par value

[6] Interest rate yields determined using a spread of 700 basis points over comparable municipal securities of varying durations.

[7] Interest rate yields determined using a spread of 600 basis points over comparable municipal securities of varying durations.

[8] Represents exchange-traded energy derivative contracts

[9] Interest rate yields used to value investment grade taxable securities based on comparable short-term taxable securities which are generally yielding less than 1%.

[10] Interest rate yields used to value investment grade taxable securities based on comparable short-term taxable securities which are generally yielding less than 1%. 6 Amortized cost reduced by other-than-temporary impairments recorded in earnings. See Note 2 for additional discussion.