Regulatory Capital Requirements - Schedule of Capital Amounts and Ratios (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
CapStar Financial Holdings, Inc.
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Total capital to risk-weighted assets, actual amount		$ 226,159	$ 222,030
Tier I capital to risk-weighted assets, actual amount		213,021	209,738
Tier I capital to average assets, actual amount		$ 213,021	$ 209,738
Total capital to risk-weighted assets, actual ratio		12.64%	12.84%
Tier I capital to risk-weighted assets, actual ratio		11.90%	12.13%
Tier I capital to average assets, actual ratio		10.97%	11.06%
Total capital to risk-weighted assets, minimum capital requirement amount	[1]	$ 143,152	$ 138,336
Tier I capital to risk-weighted assets, minimum capital requirement amount	[1]	107,364	103,752
Tier I capital to average assets, minimum capital requirement amount	[1]	$ 77,671	$ 75,867
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		$ 204,021	$ 200,738
Tier I capital to risk-weighted assets, actual ratio		11.40%	11.61%
Tier I capital to risk-weighted assets, minimum capital requirement amount	[1]	$ 80,523	$ 77,814
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		$ 207,479	$ 201,972
Tier I capital to risk-weighted assets, actual amount		194,341	189,680
Tier I capital to average assets, actual amount		$ 194,341	$ 189,680
Total capital to risk-weighted assets, actual ratio		11.60%	11.68%
Tier I capital to risk-weighted assets, actual ratio		10.87%	10.97%
Tier I capital to average assets, actual ratio		10.01%	10.01%
Total capital to risk-weighted assets, minimum capital requirement amount	[1]	$ 143,091	$ 138,294
Tier I capital to risk-weighted assets, minimum capital requirement amount	[1]	107,318	103,721
Tier I capital to average assets, minimum capital requirement amount	[1]	$ 77,642	$ 75,828
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]	$ 178,864	$ 172,868
Tier I capital to risk-weighted assets, minimum to be well capitalized amount	[2]	143,091	138,294
Tier I capital to average assets, minimum to be well capitalized amount	[2]	$ 97,053	$ 94,785
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		$ 177,841	$ 173,180
Tier I capital to risk-weighted assets, actual ratio		9.94%	10.02%
Tier I capital to risk-weighted assets, minimum capital requirement amount	[1]	$ 80,489	$ 77,791
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]	$ 116,261	$ 112,364
Tier I capital to risk-weighted assets, minimum to be well capitalized ratio	[2]	6.50%	6.50%

[1] For the calendar year 2019, 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 2.5% 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.