Financial Information - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
Product Information [Line Items]
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12			$ 67	$ 0
Revenue, performance obligation, description of timing	one year or less
Available-for-sale Securities | Accounting Standards Update 2016-01
Product Information [Line Items]
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12		$ (265,900)
Other Assets | Accounting Standards Update 2016-01
Product Information [Line Items]
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12		$ 265,900
Non-Interest income
Product Information [Line Items]
Accounts receivable	$ 9,000
Pro Forma | Accounting Standards Update 2016-02
Product Information [Line Items]
Lease assets	185,000
Lease liabilities	195,000
Pro Forma | Retained Earnings | Accounting Standards Update 2016-02
Product Information [Line Items]
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12	$ 3,000