Accounting Policies and Basis of Presentation Accounting Policies and Basis of Presentation (Recently Adopted Accounting Standards) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Oct. 29, 2019	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock repurchased during period (in shares)		295,900	125,000	744,400	125,000
Stock repurchased during period		$ 135,400	$ 58,086	$ 325,900	$ 58,086
Stock repurchased during period (in USD per share)		$ 457.50	$ 464.68	$ 437.84	$ 464.68
Operating lease assets		$ 79,857		$ 79,857
Present value of lease liabilities		79,234		79,234
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease assets							$ 70,700
Present value of lease liabilities							71,800
Right-of-use asset previously classified as capital lease							9,100
Liability previously classified as capital lease							8,300
Accounting Standards Update 2016-02 | HomeBancorp Inc.
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Present value of lease liabilities							$ 1,100
Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of shares authorized to be repurchased (in shares)						500,000
Stock repurchased during period (in shares)	146,100
Stock repurchased during period	$ 69,100
Stock repurchased during period (in USD per share)	$ 472.94
Cloud Computing Arrangement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred costs		$ 3,700		$ 3,700
Repurchases From Ella Ann Holding
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of shares authorized to be repurchased (in shares)		250,000		250,000
Stock repurchased during period (in shares)		50,000
Repurchases From Ella Ann Holding | Subsequent Event
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock repurchased during period (in shares)	50,000