Fair Value Measurements Fair Value Measurements, Financial Instruments Measured On a Recurring Basis, Quantitative Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	$ 8,978,945		$ 10,147,162
Municipal and other tax-exempt securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	63,557		73,775
Other debt securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	9,212		35,241
Equity securities and mutual funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	19,444		21,328
Fair Value, Inputs, Level 3 [Member] | Available for sale Securities [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	10,870		18,695
Fair Value Inputs, Asset, Amortized Cost	10,805		18,624
Fair Value Inputs, Asset, Fair Value	10,093		17,805
Fair Value Measurements, Valuation Techniques	Discounted cash flows	[1]	Discounted cash flows	[1]
Fair Value Measurements, Significant Unobservable Input	Interest rate spread		Interest rate spread
Fair Value Inputs, Assets, Discount Rate, Minimum	4.96%	[2]	4.97%	[3]
Fair Value Inputs, Assets, Discount Rate, Maximum	5.26%	[2]	5.27%	[3]
Fair Value Inputs, Assets, Discount Rate, Weighted Average	5.21%	[2]	5.16%	[3]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value or Principal Balance, Minimum	92.65%	[4]	95.02%	[4]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value or Principal Balance, Maximum	94.32%	[4]	95.50%	[4]
Fair Value Inputs, Asset, Fair Value As Percentage of Par Value or Unpaid Principal Balance, Weighted Average	93.09%	[4]	95.24%	[4]
Investment Grade Tax Exempt Securities Yield Spread Over Comparable Securities Minimum	488		467
Investment Grade Tax Exempt Securities Yield Spread Over Comparable Securities Maximum	516		518
Fair Value, Inputs, Level 3 [Member] | Available for sale Securities [Member] | Other debt securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Asset, Par Value	4,400		4,900
Fair Value Inputs, Asset, Amortized Cost	4,400		4,900
Fair Value Inputs, Asset, Fair Value	4,150		4,712
Fair Value Measurements, Valuation Techniques	Discounted cash flows	[1]	Discounted cash flows	[1]
Fair Value Measurements, Significant Unobservable Input	Interest rate spread		Interest rate spread
Fair Value Inputs, Assets, Discount Rate, Minimum	5.62%	[5]	5.67%	[5]
Fair Value Inputs, Assets, Discount Rate, Maximum	5.67%	[5]	5.67%	[5]
Fair Value Inputs, Assets, Discount Rate, Weighted Average	5.66%	[5]	5.67%	[5]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value or Principal Balance, Minimum	92.65%	[4]	96.16%	[4]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value or Principal Balance, Maximum	92.95%	[4]	96.16%	[4]
Fair Value Inputs, Asset, Fair Value As Percentage of Par Value or Unpaid Principal Balance, Weighted Average	92.77%	[4]	96.16%	[4]
Average Yields On Comparable Short-term Taxable Securities Maximum	1.00%		1.00%
Fair Value, Inputs, Level 3 [Member] | Available for sale Securities [Member] | Equity securities and mutual funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Measurements, Valuation Techniques			Publicly announced preliminary purchase price information from acquirer
Fair Value Measurements, Significant Unobservable Input			Discount for settlement uncertainty
Fair Value, Inputs, Level 3 [Member] | Residential Mortgage Loans Held for Sale [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Asset, Amortized Cost	12,468
Fair Value Inputs, Asset, Fair Value	11,856
Fair Value Measurements, Valuation Techniques	Quoted prices of loans sold in securitization transactions, with a liquidity discount applied
Fair Value Measurements, Significant Unobservable Input	Liquidity discount applied to the market value of mortgage loans qualifying for sale to U.S. government agencies
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value or Principal Balance, Minimum	95.09%
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value or Principal Balance, Maximum	95.09%
Fair Value Inputs, Asset, Fair Value As Percentage of Par Value or Unpaid Principal Balance, Weighted Average	95.90%
Fair Value, Inputs, Level 3 [Member] | Other assets - private equity funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Measurements, Valuation Techniques	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
Fair Value, Measurements, Recurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	8,978,945		10,147,162
Other assets, private equity funds	25,627		27,341
Fair Value, Measurements, Recurring [Member] | Municipal and other tax-exempt securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	63,557		73,775
Fair Value, Measurements, Recurring [Member] | Other debt securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	9,212		35,241
Fair Value, Measurements, Recurring [Member] | Equity securities and mutual funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	19,444		21,328
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	14,243		26,724
Other assets, private equity funds	25,627		27,341
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Municipal and other tax-exempt securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	10,093		17,805
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other debt securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	4,150		4,712
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Equity securities and mutual funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	0		4,207
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	8,958,770		10,119,396
Other assets, private equity funds	0		0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Municipal and other tax-exempt securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	53,464		55,970
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Other debt securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	5,062		30,529
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Equity securities and mutual funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available for sale securities	$ 14,517		$ 17,121

[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

[2] Interest rate yields used to value investment grade tax-exempt securities represent a spread of 488 to 516 basis points over average yields for comparable tax-exempt securities.

[3] Interest rate yields used to value investment grade tax-exempt securities represent a spread of 467 to 518 basis points over average yields for comparable tax-exempt securities.

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

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