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Description of Business and Summary of Significant Accounting Policies (Details)	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) depository_branch	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2019 USD ($)
Accounting Policies [Abstract]
Percentage of capital stock of the Bank held			100.00%
Number of depository branches \| depository_branch			44
Cash and Cash Equivalents
Cash reserves required by the Board of Governors of the Federal Reserve for depository institutions (in dollars)			$ 0
Premises, furniture and equipment
Valuation allowance			0	$ 380,000	$ 0
Excess tax benefit of stock-based compensation			(1,811,000)	$ (1,995,000)	$ 0
Reclassification of certain tax effects of the Tax Cuts and Jobs Act			0
Accumulated Other Comprehensive Income (Loss)
Premises, furniture and equipment
Reclassification of certain tax effects of the Tax Cuts and Jobs Act			82,000
Accumulated Retained Earnings
Premises, furniture and equipment
Reclassification of certain tax effects of the Tax Cuts and Jobs Act			$ (82,000)
Accounting Standards Update 2018-02 \| Accumulated Other Comprehensive Income (Loss)
Premises, furniture and equipment
Reclassification of certain tax effects of the Tax Cuts and Jobs Act	$ 82,000
Accounting Standards Update 2018-02 \| Accumulated Retained Earnings
Premises, furniture and equipment
Reclassification of certain tax effects of the Tax Cuts and Jobs Act		$ (82,000)
Accounting Standards Update 2016-02 \| Scenario, Forecast
Premises, furniture and equipment
Liability for future lease payments						$ 43,000,000
Right of use assets						$ 41,000,000
Core Deposits \| Minimum
Premises, furniture and equipment
Weighted average useful life (in years)			6 years
Core Deposits \| Maximum
Premises, furniture and equipment
Weighted average useful life (in years)			11 years 6 months
Premises
Premises, furniture and equipment
Estimated useful life (in years)			40 years
Furniture, fixtures and equipment
Premises, furniture and equipment
Estimated useful life (in years)			7 years
Computer and telecommunication
Premises, furniture and equipment
Estimated useful life (in years)			3 years