Investment Securities (Maturities Table) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Securities Available for Sale
One year or less	$ 10,957
Available for Sale Securities Debt Maturities Maturity Period of One Year or Less Weighted Average Yield	2.32%
Over one year through five years	$ 224,674
Available for Sale Securities Debt Maturities Maturity Period over One Year Through Five Years Weighted Average Yield	3.54%
Over five years through ten years	$ 554,884
Available for Sale Securities Debt Maturities Maturity Period over Five Years Through Ten Years Weighted Average Yield	2.92%
Over ten years	$ 3,525,385
Available for Sale Securities Debt Maturities Maturity Period over Ten Years Weighted Average Yield	2.23%
Available-for-sale	$ 4,318,688	$ 3,967,735	$ 3,838,509
Amortized Cost	$ 4,274,004	4,005,782	3,851,202
Weighted-Average Yield	2.39%
Securities Held to Maturity
One year or less	$ 87,315
Held to Maturity Securities Debt Maturities Maturity Period over One Year Through Five Years Weighted Average Yield	2.36%
Over one year through five years	$ 364,802
Held to Maturity Securities Debt Maturities Maturity Period over Five Years Through Ten Years Weighted Average Yield	2.47%
Over five years through ten years	$ 308,418
Held to Maturity Securities Debt Maturities Maturity Period over Ten Years Weighted Average Yield	3.83%
Over ten years	$ 1,785,205
Held to Maturity Securities Debt Maturities Maturity Period Weighted Average Yield	2.02%
Held-to-maturity Securities, Fair Value	$ 2,545,740	2,659,119	2,760,120
Held-to-maturity Securities, Carrying Amount	$ 2,514,161	2,674,093	2,787,513
Weighted Average Yield	2.31%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 12 months 7 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	3 years 12 months 29 days
US Government Agencies Debt Securities
Securities Available for Sale
One year or less	$ 0
Over one year through five years	2,528
Over five years through ten years	0
Over ten years	0
Available-for-sale	2,528	2,498	2,510
Amortized Cost	$ 2,500	2,500	2,500
Weighted-Average Yield	1.25%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	25,035
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	25,035	25,019	24,677
Held-to-maturity Securities, Carrying Amount	$ 25,000	25,000	25,000
Weighted Average Yield	1.43%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	2 years 8 months 2 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	3 years 12 months 29 days
US Treasury Securities
Securities Available for Sale
One year or less	$ 4,997
Over one year through five years	0
Over five years through ten years	0
Over ten years	0
Available-for-sale	4,997	5,000	5,005
Amortized Cost	$ 4,996	5,001	5,004
Weighted-Average Yield	0.31%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	0
Held-to-maturity Securities, Carrying Amount	$ 0
Weighted Average Yield	0.00%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	6 months
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	1 day
U.S. States and political subdivisions debt securities
Securities Available for Sale
One year or less	$ 5,895
Over one year through five years	75,103
Over five years through ten years	51,441
Over ten years	19,108
Available-for-sale	151,547	192,795	207,617
Amortized Cost	$ 147,807	188,829	203,449
Weighted-Average Yield	5.00%
Securities Held to Maturity
One year or less	$ 64,872
Over one year through five years	157,172
Over five years through ten years	193,355
Over ten years	140,474
Held-to-maturity Securities, Fair Value	555,873	593,656	535,603
Held-to-maturity Securities, Carrying Amount	$ 540,425	571,738	529,441
Weighted Average Yield	4.21%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	2 years 5 months 24 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	5 years 4 months 11 days
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
One year or less	$ 65
Over one year through five years	86,480
Over five years through ten years	47,116
Over ten years	726,586
Available-for-sale	860,247	906,229	960,852
Amortized Cost	$ 840,250	900,358	947,347
Weighted-Average Yield	2.98%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	21,307
Over ten years	458,522
Held-to-maturity Securities, Fair Value	479,829	509,676	559,252
Held-to-maturity Securities, Carrying Amount	$ 467,462	507,908	555,273
Weighted Average Yield	2.13%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 9 months 23 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	3 years 12 months 29 days
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
One year or less	$ 0
Over one year through five years	24,498
Over five years through ten years	123,441
Over ten years	54,109
Available-for-sale	202,048	172,109	170,338
Amortized Cost	$ 198,069	173,912	171,842
Weighted-Average Yield	2.13%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	36,522
Over five years through ten years	53,391
Over ten years	0
Held-to-maturity Securities, Fair Value	89,913	64,671	57,655
Held-to-maturity Securities, Carrying Amount	$ 87,964	64,951	57,462
Weighted Average Yield	2.14%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 12 months 14 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	2 years 7 months 17 days
Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
One year or less	$ 0
Over one year through five years	13,936
Over five years through ten years	10,705
Over ten years	2,328,377
Available-for-sale	2,353,018	2,128,320	1,949,389
Amortized Cost	$ 2,325,707	2,155,808	1,968,918
Weighted-Average Yield	2.01%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	0
Over ten years	1,058,153
Held-to-maturity Securities, Fair Value	1,058,153	1,125,534	1,232,741
Held-to-maturity Securities, Carrying Amount	$ 1,060,496	1,161,340	1,267,321
Weighted Average Yield	1.59%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 8 months 9 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	3 years 8 months 9 days
Residential Collateralized Mortgage-Backed Securities Issued by Non-US Government Agency
Securities Available for Sale
One year or less	$ 0
Over one year through five years	3
Over five years through ten years	0
Over ten years	0
Available-for-sale	3	4	5
Amortized Cost	$ 3	4	5
Weighted-Average Yield	2.98%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	0
Held-to-maturity Securities, Carrying Amount	$ 0
Weighted Average Yield	0.00%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 12 months 25 days
Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises
Securities Available for Sale
One year or less	$ 0
Over one year through five years	22,126
Over five years through ten years	103,837
Over ten years	272,648
Available-for-sale	398,611	216,319	228,438
Amortized Cost	$ 392,205	217,008	227,889
Weighted-Average Yield	2.18%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	81,237
Over five years through ten years	40,365
Over ten years	128,056
Held-to-maturity Securities, Fair Value	249,658	252,424	260,222
Held-to-maturity Securities, Carrying Amount	$ 246,509	255,359	263,741
Weighted Average Yield	1.77%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	3 years 9 months 30 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	4 years 1 month 27 days
Collateralized Loan Obligations
Securities Available for Sale
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	218,344
Over ten years	73,450
Available-for-sale	291,794	289,411	258,081
Amortized Cost	$ 297,939	297,831	259,743
Weighted-Average Yield	3.08%
Securities Held to Maturity
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	0
Held-to-maturity Securities, Carrying Amount	$ 0
Weighted Average Yield	0.00%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	6 years 5 months 24 days
Corporate debt securities
Securities Available for Sale
One year or less	$ 0
Over one year through five years	0
Over five years through ten years	0
Over ten years	51,107
Available-for-sale	51,107	52,229	53,450
Amortized Cost	$ 61,740	61,710	61,681
Weighted-Average Yield	1.37%
Securities Held to Maturity
One year or less	$ 22,443
Over one year through five years	64,836
Over five years through ten years	0
Over ten years	0
Held-to-maturity Securities, Fair Value	87,279	88,139	89,970
Held-to-maturity Securities, Carrying Amount	$ 86,305	87,797	89,275
Weighted Average Yield	2.30%
Available for Sale Securities, Debt Maturities, Weighted-Average Mat	11 years 12 months 22 days
Held to Maturity Securities Debt Maturities-Weighted Aver Maturity	2 years 3 months 12 days
Debt Securities
Securities Available for Sale
Available-for-sale	$ 4,315,900	3,964,914	3,835,685
Amortized Cost	$ 4,271,216	$ 4,002,961	$ 3,848,378