Regulatory Matters (Details)	Sep. 30, 2018	Dec. 31, 2017
Regulatory Matters
Common Equity Tier 1 required for capital adequacy purposes, ratio (as a percent)	4.50%	4.50%
Tier 1 required for capital adequacy purposes, ratio (as a percent)	6.00%	6.00%
Total capital required for capital adequacy purposes, ratio (as a percent)	8.00%	8.00%
Leverage required for capital adequacy purposes, ratio (as a percent)	4.00%	4.00%
Conservation Buffer Rule Starting January 2016
Regulatory Matters
Capital conservation buffer annual increase (as a percent)	0.625%
Conservation Buffer Rule Subsequent to 2016
Regulatory Matters
Capital conservation buffer annual increase (as a percent)	0.625%
Maximum | Conservation Buffer Rule
Regulatory Matters
Capital conservation buffer (as a percent)	2.50%