Investment Securities (Contractual Maturities and Narrative) (Details 2) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Prepayable debt securities carrying value	$ 2,607,636	$ 2,022,158	$ 2,236,144
Unrealized gain	21,143	9,225	15,695
Available-for-Sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis Rolling Maturity [Abstract]
Due in One Year or Less	126,483
Due After One Year Through Five Years	114,822
Due After Five Years Through Ten Years	171,950
Due After Ten Years	51
Available-for-Sale, Total	413,306
Available-for-Sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturities [Abstract]
Due in One Year or Less	126,672
Due After One Year Through Five Years	114,858
Due After Five Years Through Ten Years	173,137
Due After Ten Years	52
Fair Value, Total	414,719
Available-for-Sale: Amortized Cost	2,607,636
Available-for-Sale	2,619,003	2,007,942
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturity [Abstract]
Due in One Year or Less	294,760
Due After One Year Through Five Years	49,401
Amortized Cost, Total	344,161
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	296,286
Due After One Year Through Five Years	50,261
Fair Value, Total	346,547
Held-to-Maturity: Amortized Cost	3,042,294
Held-to-Maturity: Fair Value	3,062,882	3,413,994	3,894,121
Carrying value of investment securities which were pledged to secure deposits of gov't entities and repos	2,600,000	2,300,000	2,400,000
Mortgage-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Prepayable debt securities carrying value	1,973,110	842,820	925,062
Unrealized gain	18,879	3,505	3,549
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	1,973,110
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed-Securities: Fair Value	1,984,307
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed-Securities: Amortized Cost	2,698,133
Held-to-Maturity: Mortgage-Backed-Securities: Fair Value	2,716,335
Held-to-Maturity: Fair Value	2,716,335	2,725,516	3,154,684
US Government Agencies Debt 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	221,220
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed-Securities: Fair Value	219,977
Residential - Government Agencies
Debt Securities, Available-for-sale [Line Items]
Prepayable debt securities carrying value	1,164,466	189,645	233,268
Unrealized gain	11,627	1,726	3,129
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	1,164,466
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed-Securities: Fair Value	1,172,826
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed-Securities: Amortized Cost	1,067,416
Held-to-Maturity: Mortgage-Backed-Securities: Fair Value	1,075,315
Held-to-Maturity: Fair Value	1,075,315	1,813,987	2,202,913
Residential - U.S. Government Sponsored Enterprises
Debt Securities, Available-for-sale [Line Items]
Prepayable debt securities carrying value	584,272	589,311	619,795
Unrealized gain	4,363	1,779	420
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	584,272
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed-Securities: Fair Value	586,761
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed-Securities: Amortized Cost	1,546,479
Held-to-Maturity: Mortgage-Backed-Securities: Fair Value	1,557,872
Held-to-Maturity: Fair Value	1,557,872	740,166	752,968
Commercial - Government Agencies
Debt Securities, Available-for-sale [Line Items]
Prepayable debt securities carrying value	224,372	63,864	71,999
Unrealized gain	2,889	0	0
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	224,372
Available-for-sale Securities, Debt Securities Issued by Government Agencies and Mortgage-Backed-Securities: Fair Value	224,720
Held-to-Maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturities [Abstract]
Held-to-Maturity: Mortgage-Backed-Securities: Amortized Cost	84,238
Held-to-Maturity: Mortgage-Backed-Securities: Fair Value	83,148
Held-to-Maturity: Fair Value	83,148	$ 171,363	$ 198,803
Mortgage-backed, US Government Agencies and Government-Sponsored Enterprises, Municipal Debt Securities And Corporate Debt Securities
Debt Securities, Available-for-sale [Line Items]
Prepayable debt securities carrying value	1,000,000
Unrealized gain	$ 3,100