Segments - Operating Segment Information (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Operating Statistics [Line Items]
Assets	$ 8,593,684,000		$ 8,593,684,000		$ 7,622,637,000
Held for sale loans			27,600,000		31,100,000
Gross loans and deferred fees, net	6,383,874,000		6,383,874,000		5,678,194,000
Less: Allowance for credit losses	(96,323,000)		(96,323,000)		(95,427,000)
Loans, net	6,287,551,000		6,287,551,000		5,582,767,000
Goodwill and intangible assets	23,224,000		23,224,000		23,224,000
Deposits	7,001,286,000		7,001,286,000		6,455,177,000
Borrowings	418,607,000		418,607,000		193,717,000
Borrowings	294,677,000		294,677,000		120,000,000
Stockholders' equity	799,525,000		799,525,000		759,616,000
Net interest income	82,152,000	70,805,000	158,355,000	140,862,000
Provision for credit losses	3,481,000	13,330,000	8,920,000	26,411,000
Net interest income (loss) after provision for credit losses	78,671,000	57,475,000	149,435,000	114,451,000
Non-interest income	10,862,000	7,397,000	14,761,000	13,281,000
Non-interest expense	(48,531,000)	(45,431,000)	(95,460,000)	(92,328,000)
Income (loss) from continuing operations before income taxes	41,002,000	19,441,000	68,736,000	35,404,000
Income tax expense (benefit)	6,817,000	5,259,000	13,625,000	9,700,000
Income (loss) from continuing operations	34,185,000	14,182,000	55,111,000	25,704,000
Loss from discontinued operations, net	(169,000)	(221,000)	(131,000)	(443,000)
Net income (loss)	34,016,000	13,961,000	54,980,000	25,261,000
Western Alliance Bank [Member]
Operating Statistics [Line Items]
Assets	3,053,000,000	2,349,600,000	3,053,000,000	2,349,600,000
Gross loans and deferred fees, net	2,454,400,000	1,783,600,000	2,454,400,000	1,783,600,000
Less: Allowance for credit losses	(22,300,000)	(20,300,000)	(22,300,000)	(20,300,000)
Loans, net	2,432,100,000	1,763,300,000	2,432,100,000	1,763,300,000
Goodwill and intangible assets	3,000,000		3,000,000
Goodwill and intangible assets		3,800,000		3,800,000
Deposits	2,648,900,000	1,998,200,000	2,648,900,000	1,998,200,000
Borrowings	74,700,000		74,700,000
Borrowings		70,000,000		70,000,000
Stockholders' equity	262,400,000	208,700,000	262,400,000	208,700,000
Net interest income	32,530,000	24,060,000	59,165,000	47,116,000
Provision for credit losses	1,009,000	2,100,000	3,644,000	103,000
Net interest income (loss) after provision for credit losses	31,521,000	21,960,000	55,521,000	47,013,000
Non-interest income	11,300,000	1,994,000	12,704,000	3,847,000
Non-interest expense	(17,100,000)	(12,086,000)	(30,168,000)	(24,005,000)
Income (loss) from continuing operations before income taxes	25,721,000	11,868,000	38,057,000	26,855,000
Income tax expense (benefit)	5,247,000	4,091,000	9,089,000	9,263,000
Income (loss) from continuing operations	20,474,000	7,777,000	28,968,000	17,592,000
Net income (loss)	20,474,000	7,777,000	28,968,000	17,592,000
Bank of Nevada [Member]
Operating Statistics [Line Items]
Assets	3,319,200,000	2,920,200,000	3,319,200,000	2,920,200,000
Gross loans and deferred fees, net	2,416,300,000	2,002,100,000	2,416,300,000	2,002,100,000
Less: Allowance for credit losses	(55,800,000)	(60,500,000)	(55,800,000)	(60,500,000)
Loans, net	2,360,500,000	1,941,600,000	2,360,500,000	1,941,600,000
Goodwill and intangible assets	25,600,000		25,600,000
Goodwill and intangible assets		25,800,000		25,800,000
Deposits	2,619,900,000	2,430,900,000	2,619,900,000	2,430,900,000
Borrowings	200,000,000		200,000,000
Borrowings		100,000,000		100,000,000
Stockholders' equity	393,100,000	329,500,000	393,100,000	329,500,000
Net interest income	30,679,000	27,498,000	59,933,000	55,337,000
Provision for credit losses	999,000	8,747,000	1,404,000	22,229,000
Net interest income (loss) after provision for credit losses	29,680,000	18,751,000	58,529,000	33,108,000
Non-interest income	3,732,000	4,291,000	7,069,000	7,874,000
Non-interest expense	(16,034,000)	(18,140,000)	(33,925,000)	(36,970,000)
Income (loss) from continuing operations before income taxes	17,378,000	4,902,000	31,673,000	4,012,000
Income tax expense (benefit)	4,672,000	1,137,000	8,265,000	(714,000)
Income (loss) from continuing operations	12,706,000	3,765,000	23,408,000	4,726,000
Net income (loss)	12,706,000	3,765,000	23,408,000	4,726,000
Torrey Pines Bank [Member]
Operating Statistics [Line Items]
Assets	2,030,700,000	1,895,900,000	2,030,700,000	1,895,900,000
Held for sale loans			27,600,000
Gross loans and deferred fees, net	1,494,000,000	1,422,000,000	1,494,000,000	1,422,000,000
Less: Allowance for credit losses	(15,600,000)	(16,700,000)	(15,600,000)	(16,700,000)
Loans, net	1,478,500,000	1,405,300,000	1,478,500,000	1,405,300,000
Goodwill and intangible assets		200,000		200,000
Deposits	1,749,400,000	1,592,500,000	1,749,400,000	1,592,500,000
Borrowings	45,000,000		45,000,000
Borrowings		72,000,000		72,000,000
Stockholders' equity	170,100,000	161,600,000	170,100,000	161,600,000
Net interest income	20,603,000	21,374,000	41,380,000	42,610,000
Provision for credit losses	740,000	2,483,000	832,000	4,079,000
Net interest income (loss) after provision for credit losses	19,863,000	18,891,000	40,548,000	38,531,000
Non-interest income	606,000	1,079,000	1,204,000	2,256,000
Non-interest expense	(11,958,000)	(11,338,000)	(23,927,000)	(22,410,000)
Income (loss) from continuing operations before income taxes	8,511,000	8,632,000	17,825,000	18,377,000
Income tax expense (benefit)	2,669,000	3,340,000	5,668,000	7,297,000
Income (loss) from continuing operations	5,842,000	5,292,000	12,157,000	11,080,000
Net income (loss)	5,842,000	5,292,000	12,157,000	11,080,000
Other [Member]
Operating Statistics [Line Items]
Assets	1,124,800,000	800,700,000	1,124,800,000	800,700,000
Gross loans and deferred fees, net	62,100,000		62,100,000
Less: Allowance for credit losses	(2,600,000)		(2,600,000)
Loans, net	59,500,000		59,500,000
Goodwill and intangible assets		4,100,000		4,100,000
Borrowings	123,900,000		123,900,000
Borrowings		74,000,000		74,000,000
Stockholders' equity	817,800,000	680,400,000	817,800,000	680,400,000
Net interest income	(1,660,000)	(2,127,000)	(2,123,000)	(4,201,000)
Provision for credit losses	733,000		3,040,000
Net interest income (loss) after provision for credit losses	(2,393,000)	(2,127,000)	(5,163,000)	(4,201,000)
Non-interest income	(294,000)	1,929,000	1,411,000	3,893,000
Non-interest expense	(7,921,000)	(5,763,000)	(15,067,000)	(13,532,000)
Income (loss) from continuing operations before income taxes	(10,608,000)	(5,961,000)	(18,819,000)	(13,840,000)
Income tax expense (benefit)	(5,771,000)	(3,309,000)	(9,397,000)	(6,146,000)
Income (loss) from continuing operations	(4,837,000)	(2,652,000)	(9,422,000)	(7,694,000)
Loss from discontinued operations, net	(169,000)	(221,000)	(131,000)	(443,000)
Net income (loss)	(5,006,000)	(2,873,000)	(9,553,000)	(8,137,000)
Inter-segment eliminations [Member]
Operating Statistics [Line Items]
Assets	(934,000,000)	(802,800,000)	(934,000,000)	(802,800,000)
Gross loans and deferred fees, net	(43,000,000)	(42,900,000)	(43,000,000)	(42,900,000)
Loans, net	(43,000,000)	(42,900,000)	(43,000,000)	(42,900,000)
Deposits	(16,900,000)	(20,200,000)	(16,900,000)	(20,200,000)
Borrowings	(25,000,000)		(25,000,000)
Borrowings		(12,000,000)		(12,000,000)
Stockholders' equity	(843,900,000)	(708,100,000)	(843,900,000)	(708,100,000)
Non-interest income	(4,482,000)	(1,896,000)	(7,627,000)	(4,589,000)
Non-interest expense	4,482,000	1,896,000	7,627,000	4,589,000
Consolidated Company [Member]
Operating Statistics [Line Items]
Assets	8,593,700,000	7,163,600,000	8,593,700,000	7,163,600,000
Held for sale loans			27,600,000
Gross loans and deferred fees, net	6,383,900,000	5,164,800,000	6,383,900,000	5,164,800,000
Less: Allowance for credit losses	(96,300,000)	(97,500,000)	(96,300,000)	(97,500,000)
Loans, net	6,287,600,000	5,067,300,000	6,287,600,000	5,067,300,000
Goodwill and intangible assets	28,600,000		28,600,000
Goodwill and intangible assets		33,900,000		33,900,000
Deposits	7,001,300,000	6,001,400,000	7,001,300,000	6,001,400,000
Borrowings	418,600,000		418,600,000
Borrowings		303,500,000		303,500,000
Stockholders' equity	799,500,000	672,100,000	799,500,000	672,100,000
Net interest income	82,152,000	70,805,000	158,355,000	140,862,000
Provision for credit losses	3,481,000	13,330,000	8,920,000	26,411,000
Net interest income (loss) after provision for credit losses	78,671,000	57,475,000	149,435,000	114,451,000
Non-interest income	10,862,000	7,397,000	14,761,000	13,281,000
Non-interest expense	(48,531,000)	(45,431,000)	(95,460,000)	(92,328,000)
Income (loss) from continuing operations before income taxes	41,002,000	19,441,000	68,736,000	35,404,000
Income tax expense (benefit)	6,817,000	5,259,000	13,625,000	9,700,000
Income (loss) from continuing operations	34,185,000	14,182,000	55,111,000	25,704,000
Loss from discontinued operations, net	(169,000)	(221,000)	(131,000)	(443,000)
Net income (loss)	$ 34,016,000	$ 13,961,000	$ 54,980,000	$ 25,261,000