Portfolio loans - Adverse Loan Grading (Details) $ in Billions	6 Months Ended
	Jun. 30, 2018 USD ($) item
Loans
The number of components contained in the general portion of the Company's allowance for loan losses	2
Number of quarters whose historical average is used to calculate the historical loss ratio	20
Balance of acquired loans | $	$ 1.5
Substandard
Loans
Additional reserve (as a percent)	3.00%
Special Mention
Loans
Additional reserve (as a percent)	1.00%