Investment Securities - Schedule Of Investments Within The Available For Sale Portfolio That Had Unrealized Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	$ 2,677,574	$ 2,514,856
Less than 12 months, Unrealized Losses	(83,377)	(12,243)
12 months or longer, Fair Value	1,696,331	1,164,073
12 months or longer, Unrealized Losses	(97,300)	(35,674)
Total Fair Value	4,373,905	3,678,929
Total Unrealized Losses	(180,677)	(47,917)
U.S. treasuries
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	0	99
Less than 12 months, Unrealized Losses	0	(1)
12 months or longer, Fair Value	98	0
12 months or longer, Unrealized Losses	(2)	0
Total Fair Value	98	99
Total Unrealized Losses	(2)	(1)
Government agency issued MBS
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	1,500,317	1,455,476
Less than 12 months, Unrealized Losses	(52,607)	(4,738)
12 months or longer, Fair Value	732,470	331,900
12 months or longer, Unrealized Losses	(39,604)	(8,866)
Total Fair Value	2,232,787	1,787,376
Total Unrealized Losses	(92,211)	(13,604)
Government agency issued CMO
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	1,000,444	1,043,987
Less than 12 months, Unrealized Losses	(28,664)	(7,464)
12 months or longer, Fair Value	963,763	832,173
12 months or longer, Unrealized Losses	(57,694)	(26,808)
Total Fair Value	1,964,207	1,876,160
Total Unrealized Losses	(86,358)	(34,272)
Other U.S. government agencies
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	113,088
Less than 12 months, Unrealized Losses	(965)
12 months or longer, Fair Value	0
12 months or longer, Unrealized Losses	0
Total Fair Value	113,088
Total Unrealized Losses	(965)
Corporates and other debt
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	40,024
Less than 12 months, Unrealized Losses	(691)
12 months or longer, Fair Value	0
12 months or longer, Unrealized Losses	0
Total Fair Value	40,024
Total Unrealized Losses	(691)
States and municipalities
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	23,701
Less than 12 months, Unrealized Losses	(450)
12 months or longer, Fair Value	0
12 months or longer, Unrealized Losses	0
Total Fair Value	23,701
Total Unrealized Losses	$ (450)
Corporates and other debt
Schedule of Investments [Line Items]
Less than 12 months, Fair Value		15,294
Less than 12 months, Unrealized Losses		(40)
12 months or longer, Fair Value		0
12 months or longer, Unrealized Losses		0
Total Fair Value		15,294
Total Unrealized Losses		$ (40)