Estimated Fair Values (Narrative) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment securities available for sale, fair value	$ 7,350,194	$ 7,171,917
Liability transfers between Level 1 and Level 2	0
Asset transfers between Level 1 and Level 2	0
Liability transfers between Level 2 and Level 1	0
Asset transfers between Level 2 and Level 1	0
Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liability measured at fair value on nonrecurring basis	$ 0	$ 0
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discounts for collateral value estimates due to estimated holding and selling costs (percent)	10.00%	10.00%
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discounts for collateral value estimates due to estimated holding and selling costs (percent)	14.00%	14.00%