Dividend Availability and Regulatory Matters Dividend Availability and Regulatory Matters (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier One Leverage Capital Required for Capital Adequacy to Average Assets	6.00%
Tier One Leverage Capital	$ 2,255,588	$ 2,361,800
Tier One Leverage Capital to Average Assets	10.80%	11.75%
Tier One Leverage Capital Required to be Well Capitalized	$ 1,113,341	$ 1,107,900
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	5.00%	5.00%
Excess Tier One Leverage Capital	$ 1,142,247	$ 1,253,900
Tier One Risk Based Common Equity	$ 2,255,588	$ 2,361,800
Tier One Risk Based Common Equity to Risk Weighted Assets	10.68%	11.91%
Tier One Risk Based Common Equity Required to be Well Capitalized	$ 1,464,131	$ 1,407,743
Tier One Risk Based Common Equity Required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Excess Common Equity Tier One Capital	$ 791,457	$ 954,057
Tier One Risk Based Capital	$ 2,255,588	$ 2,361,800
Tier One Risk Based Capital to Risk Weighted Assets	10.68%	11.91%
Tier One Risk Based Capital Required to be Well Capitalized	$ 1,802,008	$ 1,732,607
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	8.00%	8.00%
Excess Tier One Risk Based Capital	$ 453,580	$ 629,193
Capital	$ 2,865,152	$ 2,978,643
Capital to Risk Weighted Assets	11.44%	12.69%
Capital Required to be Well Capitalized	$ 2,252,510	$ 2,165,759
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Excess Capital	$ 612,642	$ 812,884
Pacific Western Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier One Leverage Capital	$ 2,403,244	$ 2,574,561
Tier One Leverage Capital to Average Assets	10.13%	10.66%
Tier One Leverage Capital Required to be Well Capitalized	$ 1,112,356	$ 1,095,656
Excess Tier One Leverage Capital	1,290,888	1,478,905
Tier One Risk Based Common Equity	$ 2,403,244	$ 2,574,561
Tier One Risk Based Common Equity to Risk Weighted Assets	10.01%	10.91%
Tier One Risk Based Common Equity Required to be Well Capitalized	$ 1,462,083	$ 1,405,299
Excess Common Equity Tier One Capital	941,161	1,169,262
Tier One Risk Based Capital	$ 2,403,244	$ 2,574,561
Tier One Risk Based Capital to Risk Weighted Assets	10.01%	10.91%
Tier One Risk Based Capital Required to be Well Capitalized	$ 1,799,487	$ 1,729,599
Excess Tier One Risk Based Capital	603,757	844,962
Capital	$ 2,572,586	$ 2,742,624
Capital to Risk Weighted Assets	12.72%	13.75%
Capital Required to be Well Capitalized	$ 2,249,359	$ 2,161,999
Excess Capital	$ 323,227	$ 580,625