Estimated Fair Values (Narrative) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available for sale	$ 7,006,580	$ 6,861,293
Fair value of investment securities with nonobservable inputs	0	0
Asset transfers between Level 1 and Level 2	0	0
Asset transfers between Level 2 and Level 1	0	0
Liability transfers between Level 1 and Level 2	0	0
Liability transfers between Level 2 and Level 1	0	0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available for sale	7,006,580	6,861,293
Liability measured at fair value on nonrecurring basis	0	0
Fair Value, Measurements, Recurring \| Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available for sale	$ 0	$ 0
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effective Interest Rate For Discounted Cash Flow	2.00%	2.00%
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]
Effective Interest Rate For Discounted Cash Flow	16.00%	16.00%
Discounts for collateral value estimates due to estimated holding and selling costs (percent)	14.00%	14.00%

Discounts for Collateral Value Estimates Due to Estimated Holding and Selling Costs (Percent)

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Effective Interest Rate For Discounted Cash Flow

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Fair value of financial and nonfinancial obligations.

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