Financial Information - (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Product Information [Line Items]
Cumulative effect of new accounting principle in period of adoption			$ 67	$ 0
Non-Interest income
Product Information [Line Items]
Accounts receivable	$ 7,700
Available-for-sale Securities | Accounting Standards Update 2016-01
Product Information [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ (265,900)
Other Assets | Accounting Standards Update 2016-01
Product Information [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ 265,900