Securities (Securities Held to Maturity with Unrealized Losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses less than 12 months, Fair Value	$ 1,572,900	$ 768,589
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	29,462	6,515
Held to maturity, Losses 12 months or longer, Fair Value	1,229,621	1,055,983
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	49,587	22,096
Held to maturity, Total, Fair Value	2,802,521	1,824,572
Held to maturity, Total, Gross Unrealized Losses	79,049	28,611
U.S. Treasury and Government Agency Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses 12 months or longer, Fair Value	49,332	49,711
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	668	289
Held to maturity, Total, Fair Value	49,332	49,711
Held to maturity, Total, Gross Unrealized Losses	668	289
Municipal Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses less than 12 months, Fair Value	364,751	14,603
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	5,081	19
Held to maturity, Losses 12 months or longer, Fair Value	220,575	230,960
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	9,085	4,226
Held to maturity, Total, Fair Value	585,326	245,563
Held to maturity, Total, Gross Unrealized Losses	14,166	4,245
Residential Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses less than 12 months, Fair Value	383,695	8,815
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	7,189	99
Held to maturity, Losses 12 months or longer, Fair Value	211,897	230,277
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	7,541	2,591
Held to maturity, Total, Fair Value	595,592	239,092
Held to maturity, Total, Gross Unrealized Losses	14,730	2,690
Commercial Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses less than 12 months, Fair Value	232,031	174,882
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	9,079	744
Held to maturity, Losses 12 months or longer, Fair Value	70,133	72,499
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	5,502	3,171
Held to maturity, Total, Fair Value	302,164	247,381
Held to maturity, Total, Gross Unrealized Losses	14,581	3,915
Collateralized Mortgage Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses less than 12 months, Fair Value	592,423	570,289
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	8,113	5,653
Held to maturity, Losses 12 months or longer, Fair Value	677,684	472,536
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	26,791	11,819
Held to maturity, Total, Fair Value	1,270,107	1,042,825
Held to maturity, Total, Gross Unrealized Losses	$ 34,904	$ 17,472