Capital Ratios (Details) - USD ($) $ in Thousands	Jan. 01, 2016	Jun. 30, 2018	May 24, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 31, 2013
Total Capital (to Risk Weighted Assets), Actual Ratio (as a percent)		20.04%		19.51%
Total Capital (to Risk Weighted Assets), For Capital Adequacy Purposes Ratio (as a percent)	0.625%				2.50%
Amount of assets to exempt banks		$ 11,988,995		$ 12,184,698
CET 1 ratio		17.75%		17.11%
Tier 1 capital (to Average Assets), Actual Ratio (as a percent)		15.32%		14.62%
Tier 1 capital (to Risk Weighted Assets), Actual Ratio (as a percent)		19.32%		18.73%		6.00%
Term of mandatory compliance	4 years
Minimum
Community bank leverage ratio			8.00%
CET 1 ratio		7.00%
Tier 1 capital (to Risk Weighted Assets), Actual Ratio (as a percent)					4.50%	4.00%
Maximum
Amount of assets to exempt banks			$ 10,000,000
Community bank leverage ratio			10.00%