Investment Securities (Contractual Maturities and Narrative) (Details 2) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis Rolling Maturity[Abstract]
Due in One Year or Less	$ 82,510
Due After One Year Through Five Years	610,230
Due After Five Years Through Ten Years	144,294
Due After Ten Years	23,020
Amortized Cost, Total	860,054
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Due in One Year or Less	82,492
Due After One Year Through Five Years	609,665
Due After Five Years Through Ten Years	147,503
Due After Ten Years	23,945
Fair Value, Total	863,605
Available-for-Sale: Amortized Cost	2,200,409	$ 2,236,144
Available-for-Sale: Fair Value	2,184,187	2,232,979
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturities (Abstract]
Due in One Year or Less	284,876
Due After One Year Through Five Years	200,331
Due After Five Years Through Ten Years	235,187
Due After Ten Years	27,438
Amortized Cost, Total	747,832
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Due in One Year or Less	284,063
Due After One Year Through Five Years	200,997
Due After Five Years Through Ten Years	237,411
Due After Ten Years	28,561
Fair Value, Total	751,032
Held-to-Maturity: Amortized Cost	3,789,092
Held-to-Maturity: Fair Value	3,711,149	3,894,121
Carrying value of investment securities which were pledged to secure deposits of gov't entities and repos	2,500,000	2,400,000
Debt Securities Issued by Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Amortized Cost	438,401
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	437,875
Residential - Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Amortized Cost	220,935
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	222,541
Available-for-Sale: Amortized Cost	220,935	233,268
Available-for-Sale: Fair Value	222,541	235,360
Residential - U.S. Government-Sponsored Enterprises
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Amortized Cost	610,668
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	594,095
Available-for-Sale: Amortized Cost	610,668	619,795
Available-for-Sale: Fair Value	594,095	609,812
Commercial - Government Agencies
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Amortized Cost	70,351
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	66,071
Available-for-Sale: Amortized Cost	70,351	71,999
Available-for-Sale: Fair Value	66,071	68,747
Mortgage-Backed Securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Amortized Cost	901,954
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	882,707
Available-for-Sale: Amortized Cost	901,954	925,062
Available-for-Sale: Fair Value	882,707	913,919
Residential - Government Agencies
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	2,115,460
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	2,061,649
Held-to-Maturity: Fair Value	2,061,649	2,202,913
Residential - U.S. Government-Sponsored Enterprises
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	733,427
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	711,453
Held-to-Maturity: Fair Value	711,453	752,968
Commercial - Government Agencies
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	192,373
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	187,015
Held-to-Maturity: Fair Value	187,015	198,803
Mortgage-Backed Securities
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed Securities: Amortized Cost	3,041,260
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	2,960,117
Held-to-Maturity: Fair Value	$ 2,960,117	$ 3,154,684