Minimum Capital Requirements - Additional Information (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Banking And Thrift [Abstract]
Capital conservative current buffer percentage	1.75%	1.25%
Capital conservative buffer percentage	2.50%
Description of capital adequacy purposes	Beginning in 2016, an additional conservation buffer was added to the minimum requirements for capital adequacy purposes, subject to a three year phase-in period. As of September 30, 2018 and December 31, 2017, the buffer was 1.75% and 1.25%, respectively. The capital conservative buffer will be fully phased in January 1, 2019 at 2.50%.