Securities (Securities Held to Maturity with Unrealized Losses) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses less than 12 months, Fair Value	$ 1,675,758	$ 768,589
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	36,693	6,515
Held to maturity, Losses 12 months or longer, Fair Value	1,214,258	1,055,983
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	57,469	22,096
Held to maturity, Total, Fair Value	2,890,016	1,824,572
Held to maturity, Total, Gross Unrealized Losses	94,162	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	49,292	49,711
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	708	289
Held to maturity, Total, Fair Value	49,292	49,711
Held to maturity, Total, Gross Unrealized Losses	708	289
Municipal Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses less than 12 months, Fair Value	378,622	14,603
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	7,288	19
Held to maturity, Losses 12 months or longer, Fair Value	217,955	230,960
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	10,521	4,226
Held to maturity, Total, Fair Value	596,577	245,563
Held to maturity, Total, Gross Unrealized Losses	17,809	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	454,003	8,815
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	6,996	99
Held to maturity, Losses 12 months or longer, Fair Value	198,816	230,277
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	8,505	2,591
Held to maturity, Total, Fair Value	652,819	239,092
Held to maturity, Total, Gross Unrealized Losses	15,501	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	270,664	174,882
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	11,146	744
Held to maturity, Losses 12 months or longer, Fair Value	69,335	72,499
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	6,283	3,171
Held to maturity, Total, Fair Value	339,999	247,381
Held to maturity, Total, Gross Unrealized Losses	17,429	3,915
Collateralized Mortgage Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses less than 12 months, Fair Value	572,469	570,289
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	11,263	5,653
Held to maturity, Losses 12 months or longer, Fair Value	678,860	472,536
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	31,452	11,819
Held to maturity, Total, Fair Value	1,251,329	1,042,825
Held to maturity, Total, Gross Unrealized Losses	$ 42,715	$ 17,472