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Investment Securities (Narrative) (Details)
3 Months Ended 12 Months Ended
Dec. 31, 2015
USD ($)
securities
Sep. 30, 2015
USD ($)
Jun. 30, 2015
USD ($)
Mar. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
Sep. 30, 2014
USD ($)
Jun. 30, 2014
USD ($)
Mar. 31, 2014
USD ($)
Dec. 31, 2015
USD ($)
securities
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Schedule of Available-for-sale Securities [Line Items]                      
Transfer of available-for-sale securities to held-to-maturity securities                 $ 862,712,000 $ 0 $ 0
Available-for-sale securities, transferred to held-to-maturity, unrealized gain(loss)                 14,400,000    
Income tax expense $ 20,896,000 $ 23,842,000 $ 26,295,000 $ 23,983,000 $ 23,669,000 $ 25,034,000 $ 24,490,000 $ 21,840,000 95,016,000 95,033,000 87,230,000
Investment securities pledged as collateral $ 1,200,000,000       $ 1,100,000,000       $ 1,200,000,000 1,100,000,000  
Number of securities by a single non-goverment sponsored issuer exceeding 10% of shareholders' equity | securities 0               0    
Available-for-sale securities                      
Schedule of Available-for-sale Securities [Line Items]                      
Income tax expense                 $ 122,000 $ 194,000 $ 1,200,000