Income Taxes - (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Federal tax rate	24.50%				35.00%
Provision for income taxes	$ 5.0
Increase in provision for income taxes	$ 13.6
Great Western Bank
Income Tax Contingency [Line Items]
Effective income tax percentage	49.50%	30.80%
Forecast
Income Tax Contingency [Line Items]
Federal tax rate				24.50%
Forecast | Great Western Bank
Income Tax Contingency [Line Items]
Effective income tax percentage			26.00%
Internal Revenue Service (IRS)
Income Tax Contingency [Line Items]
Income tax payable	$ 4.6				$ 4.6
Income taxes receivable		$ 4.6