Minimum Capital Requirements - Additional Information (Details)	6 Months Ended	12 Months Ended
	Jun. 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 June 30, 2018 and December 31, 2017, the buffer was 1.75 percent and 1.25 percent, respectively. The capital conservative buffer will be fully phased in January 1, 2019 at 2.5 percent.