Regulatory Capital Requirements - Schedule of Capital Amounts and Ratios (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
CapStar Financial Holdings, Inc.
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital to risk-weighted assets, actual amount		$ 155,426	$ 149,616
Tier I capital to risk-weighted assets, actual amount		141,125	137,909
Tier I capital to average assets, actual amount		$ 141,125	$ 137,909
Total capital to risk-weighted assets, actual ratio		12.42%	12.60%
Tier I capital to risk-weighted assets, actual ratio		11.28%	11.61%
Tier I capital to average assets, actual ratio		10.36%	10.46%
Total capital to risk-weighted assets, minimum capital requirement amount	[1]	$ 100,130	$ 95,028
Tier I capital to risk-weighted assets, minimum capital requirement amount	[1]	75,098	71,271
Tier I capital to average assets, minimum capital requirement amount	[1]	$ 54,464	$ 52,727
Total capital to risk-weighted assets, minimum capital requirement ratio	[1]	8.00%	8.00%
Tier I capital to risk-weighted assets, minimum capital requirement ratio	[1]	6.00%	6.00%
Tier I capital to average assets, minimum capital requirement ratio	[1]	4.00%	4.00%
CapStar Financial Holdings, Inc. | Common Stock
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Tier I capital to risk-weighted assets, actual amount		$ 132,417	$ 129,528
Tier I capital to risk-weighted assets, actual ratio		10.58%	10.90%
Tier I capital to risk-weighted assets, minimum capital requirement amount	[1]	$ 56,323	$ 53,453
Tier I capital to risk-weighted assets, minimum capital requirement ratio	[1]	4.50%	4.50%
CapStar Bank
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital to risk-weighted assets, actual amount		$ 141,843	$ 126,718
Tier I capital to risk-weighted assets, actual amount		127,542	115,011
Tier I capital to average assets, actual amount		$ 127,542	$ 115,011
Total capital to risk-weighted assets, actual ratio		11.33%	10.67%
Tier I capital to risk-weighted assets, actual ratio		10.19%	9.68%
Tier I capital to average assets, actual ratio		9.37%	8.72%
Total capital to risk-weighted assets, minimum capital requirement amount	[1]	$ 100,118	$ 95,028
Tier I capital to risk-weighted assets, minimum capital requirement amount	[1]	75,088	71,271
Tier I capital to average assets, minimum capital requirement amount	[1]	$ 54,463	$ 52,727
Total capital to risk-weighted assets, minimum capital requirement ratio	[1]	8.00%	8.00%
Tier I capital to risk-weighted assets, minimum capital requirement ratio	[1]	6.00%	6.00%
Tier I capital to average assets, minimum capital requirement ratio	[1]	4.00%	4.00%
Total capital to risk-weighted assets, minimum to be well capitalized amount	[2]	$ 125,147	$ 118,785
Tier I capital to risk-weighted assets, minimum to be well capitalized amount	[2]	100,118	95,028
Tier I capital to average assets, minimum to be well capitalized amount	[2]	$ 68,079	$ 65,909
Total capital to risk-weighted assets, minimum to be well capitalized ratio	[2]	10.00%	10.00%
Tier I capital to risk-weighted assets, minimum to be well capitalized ratio	[2]	8.00%	8.00%
Tier I capital to average assets, minimum to be well capitalized ratio	[2]	5.00%	5.00%
CapStar Bank | Common Stock
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Tier I capital to risk-weighted assets, actual amount		$ 111,334	$ 99,130
Tier I capital to risk-weighted assets, actual ratio		8.90%	8.35%
Tier I capital to risk-weighted assets, minimum capital requirement amount	[1]	$ 56,316	$ 53,453
Tier I capital to risk-weighted assets, minimum capital requirement ratio	[1]	4.50%	4.50%
Tier I capital to risk-weighted assets, minimum to be well capitalized amount	[2]	$ 81,346	$ 77,210
Tier I capital to risk-weighted assets, minimum to be well capitalized ratio	[2]	6.50%	6.50%

[1] For the calendar year 2017, the Company must maintain a capital conservation buffer of Tier 1 common equity capital in excess of minimum risk-based capital ratios by at least 1.25% to avoid limits on capital distributions and certain discretionary bonus payments to executive officers and similar employees.

[2] For the Company to be well-capitalized, the Bank must be well-capitalized and the Company must not be subject to any written agreement, order, capital directive, or prompt corrective action directive issued by the Federal Reserve to meet and maintain a specific capital level for any capital measure.