Deposits - Maturity Distribution of Time Certificates of Deposits (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Maturity distribution of time certificates of deposit
Three months or less	$ 2,077,039
Over three through six months	513,273
Over six through twelve months	869,629
One to two years	233,077
Two to three years	152,174
Three to four years	130,940
Four to five years	89,467
Thereafter	127
Total	4,065,726
Under $250,000
Maturity distribution of time certificates of deposit
Three months or less	278,084
Over three through six months	172,688
Over six through twelve months	397,408
One to two years	130,272
Two to three years	98,989
Three to four years	93,983
Four to five years	57,346
Thereafter	127
Total	1,228,897
$250,000 or More
Maturity distribution of time certificates of deposit
Three months or less	1,798,955
Over three through six months	340,585
Over six through twelve months	472,221
One to two years	102,805
Two to three years	53,185
Three to four years	36,957
Four to five years	32,121
Total	$ 2,836,829