Regulatory Capital Requirements - Summary of Actual Capital Amount and Ratio (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Capital Required for Capital Adequacy to Risk Weighted Assets	8.00%	8.00%
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	8.00%	6.00%
Tier 1 Capital Ratio	6.00%	4.00%
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	5.00%	5.00%
Common Equity Tier One Required To Be Well Capitalized	6.50%
Tier 1 Leverage Ratio	4.00%	4.00%
Common Equity Tier One Required For Capital Adequacy	4.50%
Consolidated Company [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital	$ 1,603,472	$ 1,119,618
Tier 1 Capital	1,341,011	1,007,278
Risk-Weighted Assets	13,193,563	9,555,390
Tangible Average Assets	$ 13,683,148	$ 10,367,575
Capital to Risk Weighted Assets	12.20%	11.70%
Tier One Risk Based Capital to Risk Weighted Assets	10.20%	10.50%
Tier One Leverage Capital to Average Assets	9.80%	9.70%
Common Equity Tier One Risk-Based Capital Ratio	9.70%
Western Alliance Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital	$ 1,485,070	$ 1,057,253
Tier 1 Capital	1,213,304	945,687
Risk-Weighted Assets	13,073,394	9,435,459
Tangible Average Assets	$ 13,561,251	$ 10,232,297
Capital to Risk Weighted Assets	11.40%	11.20%
Tier One Risk Based Capital to Risk Weighted Assets	9.30%	10.00%
Tier One Leverage Capital to Average Assets	9.00%	9.20%
Common Equity Tier One Risk-Based Capital Ratio	9.30%