Investment Securities (Contractual Maturities and Narrative) (Details 2) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis Rolling Maturity[Abstract]
Due in One Year or Less	$ 79,756
Due After One Year Through Five Years	622,846
Due After Five Years Through Ten Years	164,664
Due After Ten Years	25,501
Amortized Cost, Total	892,767
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Due in One Year or Less	80,005
Due After One Year Through Five Years	628,796
Due After Five Years Through Ten Years	172,362
Due After Ten Years	27,186
Fair Value, Total	908,349
Available-for-Sale: Amortized Cost	2,308,112	$ 2,183,943
Available-for-Sale: Fair Value	2,322,668	2,186,041
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturities (Abstract]
Due in One Year or Less	240,105
Due After One Year Through Five Years	255,274
Due After Five Years Through Ten Years	254,585
Due After Ten Years	38,244
Amortized Cost, Total	788,208
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Due in One Year or Less	239,988
Due After One Year Through Five Years	257,753
Due After Five Years Through Ten Years	262,701
Due After Ten Years	41,016
Fair Value, Total	801,458
Held-to-Maturity: Amortized Cost	3,960,598
Held-to-Maturity: Fair Value	3,960,956	3,827,527
Carrying value of investment securities which were pledged to secure deposits of gov't entities and repos	2,700,000	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	445,654
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	448,664
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	247,418
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	249,976
Available-for-Sale: Amortized Cost	247,418	240,412
Available-for-Sale: Fair Value	249,976	243,844
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	646,013
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	642,113
Available-for-Sale: Amortized Cost	646,013	511,234
Available-for-Sale: Fair Value	642,113	506,987
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	76,260
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	73,566
Available-for-Sale: Amortized Cost	76,260	89,544
Available-for-Sale: Fair Value	73,566	85,517
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	969,691
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed Securities: Fair Value	965,655
Available-for-Sale: Amortized Cost	969,691	841,190
Available-for-Sale: Fair Value	965,655	836,348
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	2,168,568
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	2,162,036
Held-to-Maturity: Fair Value	2,162,036	1,936,782
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	795,992
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	790,711
Held-to-Maturity: Fair Value	790,711	742,647
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	207,830
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	206,751
Held-to-Maturity: Fair Value	206,751	230,364
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	3,172,390
Held-to-Maturity: Mortgage-Backed Securities: Fair Value	3,159,498
Held-to-Maturity: Fair Value	$ 3,159,498	$ 2,909,793