Estimated Fair Values (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012
Estimated Fair Values [Line Items]
Total investment securities available for sale, fair value	$ 5,161,585	[1]	$ 5,226,228	[2]
Liability transfers between Level 1 and Level 2	0				0
Asset transfers between Level 1 and Level 2	0				0
Liability transfers between Level 2 and Level 1	0				0
Asset transfers between Level 2 and Level 1	0				0
Fair value of investment securities with nonobservable inputs	0		0
Fair Value, Measurements, Recurring
Estimated Fair Values [Line Items]
Liability measured at fair value on nonrecurring basis	0		0		0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Estimated Fair Values [Line Items]
Total investment securities available for sale, fair value	0		0		0
Fair Value, Measurements, Recurring | Fair Value
Estimated Fair Values [Line Items]
Total investment securities available for sale, fair value	$ 5,161,585		$ 5,226,228
Minimum
Estimated Fair Values [Line Items]
Discounts for collateral value estimates due to estimated holding and selling costs (percent)	10.00%
Maximum
Estimated Fair Values [Line Items]
Discounts for collateral value estimates due to estimated holding and selling costs (percent)	14.00%

[1] Unaudited

[2] Derived from 2012 Annual Report on Form 10-K.