Securities (Securities Held to Maturity with Unrealized Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses less than 12 months, Fair Value	$ 1,652,124	$ 768,589
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	28,032	6,515
Held to maturity, Losses 12 months or longer, Fair Value	1,008,026	1,055,983
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	36,528	22,096
Held to maturity, Total, Fair Value	2,660,150	1,824,572
Held to maturity, Total, Gross Unrealized Losses	64,560	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	49,424	49,711
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	576	289
Held to maturity, Total, Fair Value	49,424	49,711
Held to maturity, Total, Gross Unrealized Losses	576	289
Municipal Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses less than 12 months, Fair Value	341,011	14,603
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	4,530	19
Held to maturity, Losses 12 months or longer, Fair Value	221,438	230,960
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	8,923	4,226
Held to maturity, Total, Fair Value	562,449	245,563
Held to maturity, Total, Gross Unrealized Losses	13,453	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	403,985	8,815
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	4,090	99
Held to maturity, Losses 12 months or longer, Fair Value	224,200	230,277
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	6,534	2,591
Held to maturity, Total, Fair Value	628,185	239,092
Held to maturity, Total, Gross Unrealized Losses	10,624	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	234,468	174,882
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	6,845	744
Held to maturity, Losses 12 months or longer, Fair Value	71,098	72,499
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	4,555	3,171
Held to maturity, Total, Fair Value	305,566	247,381
Held to maturity, Total, Gross Unrealized Losses	11,400	3,915
Collateralized Mortgage Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held to maturity, Losses less than 12 months, Fair Value	672,660	570,289
Held to maturity, Losses less than 12 months, Gross Unrealized Losses	12,567	5,653
Held to maturity, Losses 12 months or longer, Fair Value	441,866	472,536
Held to maturity, Losses 12 months or longer, Gross Unrealized Losses	15,940	11,819
Held to maturity, Total, Fair Value	1,114,526	1,042,825
Held to maturity, Total, Gross Unrealized Losses	$ 28,507	$ 17,472