Fair Value of Assets and Liabilities (Financial instruments not recorded at FV on recurring basis) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Financial Instruments - Assets
Investment Securities Held-to-Maturity, Carrying Amount	$ 4,777,494	$ 4,744,519
Investment Securities Held-to-Maturity, Fair Value	4,774,032	4,697,587
Loans, Carrying Amount	5,841,620	5,707,133
Loans, Fair Value	6,248,862	6,062,147
Financial Instruments - Liabilities
Time Deposits, Carrying Amount	1,471,623	1,317,770
Time Deposits, Fair Value	1,476,282	1,322,967
Securities Sold Under Agreements to Repurchase, Carrying Amount	797,213	770,049
Securities Sold Under Agreements to Repurchase, Fair Value	872,907	846,193
Long-Term Debt, Carrying Amount	165,877	165,877
Long-Term Debt, Fair Value	167,026	167,049
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial Instruments - Assets
Investment Securities Held-to-Maturity, Fair Value	497,853	433,365
Loans, Fair Value	0	0
Financial Instruments - Liabilities
Time Deposits, Fair Value	0	0
Securities Sold Under Agreements to Repurchase, Fair Value	0	0
Long-Term Debt, Fair Value	0	0
Significant Other Observable Inputs (Level 2)
Financial Instruments - Assets
Investment Securities Held-to-Maturity, Fair Value	4,276,179	4,264,222
Loans, Fair Value	0	0
Financial Instruments - Liabilities
Time Deposits, Fair Value	1,476,282	1,322,967
Securities Sold Under Agreements to Repurchase, Fair Value	872,907	846,193
Long-Term Debt, Fair Value	167,026	167,049
Significant Other Unobservable Inputs (Level 3)
Financial Instruments - Assets
Investment Securities Held-to-Maturity, Fair Value	0	0
Loans, Fair Value	6,248,862	6,062,147
Financial Instruments - Liabilities
Time Deposits, Fair Value	0	0
Securities Sold Under Agreements to Repurchase, Fair Value	0	0
Long-Term Debt, Fair Value	$ 0	$ 0