Investment Securities (Amortization Cost, Gross Unrealized Gains/Losses, and Fair Value) (Details 1) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-Sale:
Available-for-Sale: Amortized Cost	$ 2,200,409	$ 2,236,144
Available-for-Sale: Gross Unrealized Gains	11,746	15,695
Available-for-Sale: Gross Unrealized Losses	(27,968)	(18,860)
Available-for-Sale	2,184,187	2,232,979
Held-to-maturity Securities [Abstract]
Held-to-Maturity: Amortized Cost	3,789,092	3,928,170
Held-to-Maturity: Gross Unrealized Gains	14,392	20,949
Held-to-Maturity: Gross Unrealized Losses	(92,335)	(54,998)
Held-to-Maturity: Fair Value	3,711,149	3,894,121
Debt Securities Issued by the U.S. Treasury and Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	438,951	424,912
Available-for-Sale: Gross Unrealized Gains	1,363	2,053
Available-for-Sale: Gross Unrealized Losses	(1,904)	(1,035)
Available-for-Sale	438,410	425,930
Debt Securities Issued by States and Political Subdivisions
Available-for-Sale:
Available-for-Sale: Amortized Cost	606,507	618,167
Available-for-Sale: Gross Unrealized Gains	7,119	9,894
Available-for-Sale: Gross Unrealized Losses	(1,622)	(1,042)
Available-for-Sale	612,004	627,019
Debt Securities Issued by Corporations
Available-for-Sale:
Available-for-Sale: Amortized Cost	252,997	268,003
Available-for-Sale: Gross Unrealized Gains	69	199
Available-for-Sale: Gross Unrealized Losses	(2,000)	(2,091)
Available-for-Sale	251,066	266,111
Residential - Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	220,935	233,268
Available-for-Sale: Gross Unrealized Gains	2,710	3,129
Available-for-Sale: Gross Unrealized Losses	(1,104)	(1,037)
Available-for-Sale	222,541	235,360
Residential - U.S. Government-Sponsored Enterprises
Available-for-Sale:
Available-for-Sale: Amortized Cost	610,668	619,795
Available-for-Sale: Gross Unrealized Gains	485	420
Available-for-Sale: Gross Unrealized Losses	(17,058)	(10,403)
Available-for-Sale	594,095	609,812
Commercial - Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	70,351	71,999
Available-for-Sale: Gross Unrealized Gains	0	0
Available-for-Sale: Gross Unrealized Losses	(4,280)	(3,252)
Available-for-Sale	66,071	68,747
Mortgage-Backed Securities
Available-for-Sale:
Available-for-Sale: Amortized Cost	901,954	925,062
Available-for-Sale: Gross Unrealized Gains	3,195	3,549
Available-for-Sale: Gross Unrealized Losses	(22,442)	(14,692)
Available-for-Sale	882,707	913,919
Debt Securities Issued by the U.S. Treasury and Government Agencies
Held-to-maturity Securities [Abstract]
Held-to-Maturity: Amortized Cost	394,703	375,074
Held-to-Maturity: Gross Unrealized Gains	0	18
Held-to-Maturity: Gross Unrealized Losses	(1,906)	(1,451)
Held-to-Maturity: Fair Value	392,797	373,641
Debt Securities Issued by States and Political Subdivisions
Held-to-maturity Securities [Abstract]
Held-to-Maturity: Amortized Cost	237,539	238,504
Held-to-Maturity: Gross Unrealized Gains	7,511	9,125
Held-to-Maturity: Gross Unrealized Losses	0	0
Held-to-Maturity: Fair Value	245,050	247,629
Debt Securities Issued by Corporations
Held-to-maturity Securities [Abstract]
Held-to-Maturity: Amortized Cost	115,590	119,635
Held-to-Maturity: Gross Unrealized Gains	0	123
Held-to-Maturity: Gross Unrealized Losses	(2,405)	(1,591)
Held-to-Maturity: Fair Value	113,185	118,167
Residential - Government Agencies
Held-to-maturity Securities [Abstract]
Held-to-Maturity: Amortized Cost	2,115,460	2,229,985
Held-to-Maturity: Gross Unrealized Gains	6,437	9,975
Held-to-Maturity: Gross Unrealized Losses	(60,248)	(37,047)
Held-to-Maturity: Fair Value	2,061,649	2,202,913
Residential - U.S. Government-Sponsored Enterprises
Held-to-maturity Securities [Abstract]
Held-to-Maturity: Amortized Cost	733,427	763,312
Held-to-Maturity: Gross Unrealized Gains	321	911
Held-to-Maturity: Gross Unrealized Losses	(22,295)	(11,255)
Held-to-Maturity: Fair Value	711,453	752,968
Commercial - Government Agencies
Held-to-maturity Securities [Abstract]
Held-to-Maturity: Amortized Cost	192,373	201,660
Held-to-Maturity: Gross Unrealized Gains	123	797
Held-to-Maturity: Gross Unrealized Losses	(5,481)	(3,654)
Held-to-Maturity: Fair Value	187,015	198,803
Mortgage-Backed Securities
Held-to-maturity Securities [Abstract]
Held-to-Maturity: Amortized Cost	3,041,260	3,194,957
Held-to-Maturity: Gross Unrealized Gains	6,881	11,683
Held-to-Maturity: Gross Unrealized Losses	(88,024)	(51,956)
Held-to-Maturity: Fair Value	$ 2,960,117	$ 3,154,684

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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