Accounting Policies and Basis of Presentation - Summary of Significant Accounting Policies (Details) $ in Thousands		12 Months Ended
	Nov. 01, 2019 USD ($)	Dec. 31, 2019 USD ($) state reportablesegment branch	Jan. 01, 2020 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Property, Plant and Equipment [Line Items]
Number of branches | branch		574
Number of states in which entity operates | state		19
Number of reportable segments | reportablesegment		1
Non-marketable securities		$ 12,500			$ 2,500
FHLB restricted stock		43,000			25,300
Amortization Method Qualified Affordable Housing Project Investments		167,800			147,300
Right-of-use asset		77,115
Present value of lease liabilities		76,746
Deferred Tax Assets, Gross		(131,934)			(134,739)
Total assets		39,824,496			35,408,629
Retained earnings		3,658,197			$ 3,218,551
Accounting Standards Update 2016-02
Property, Plant and Equipment [Line Items]
Right-of-use asset				$ 70,700
Present value of lease liabilities				71,800
Right-of-use asset previously classified as capital lease				8,800
Finance lease liability previously classified as capital lease				8,300
Accounting Standards Update 2018-15
Property, Plant and Equipment [Line Items]
Deferred Implementation Costs, Net, Related To Cloud Computing		$ 5,700
Accounting Standards Update 2019-04
Property, Plant and Equipment [Line Items]
Debt Securities, Held-to-maturity, Transfer, Amount	$ 2,080,000
Debt Instrument, Fair Value Disclosure	2,150,000
Other Comprehensive Income (Loss) Reclassification Adjustment Of Unrealized Losses From AOCI For Securities Available For Sale Transferred To Held To Maturity Net Of Tax	72,500
Debt Securities, Held-to-maturity, Transfer, Unrealized Gain (Loss)	1,600
Deferred Tax Assets, Gross	16,700
Total assets	57,400
Retained earnings	$ 57,400
Minimum
Property, Plant and Equipment [Line Items]
Useful life (years)		5 years
Operating lease renewal term (years)		1 year
Maximum
Property, Plant and Equipment [Line Items]
Useful life (years)		12 years
Operating lease renewal term (years)		25 years
HomeBancorp Inc. | Accounting Standards Update 2016-02
Property, Plant and Equipment [Line Items]
Present value of lease liabilities				$ 1,100
Scenario, Forecast | Accounting Standards Updated 2016-13 | Minimum
Property, Plant and Equipment [Line Items]
Retained earnings			$ 32,000
Allowance For Credit Losses, Increase (Decrease), Percentage			15.00%
Scenario, Forecast | Accounting Standards Updated 2016-13 | Maximum
Property, Plant and Equipment [Line Items]
Retained earnings			$ 42,000
Allowance For Credit Losses, Increase (Decrease), Percentage			20.00%