Dividend Availability and Regulatory Matters Dividend Availability and Regulatory Matters (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
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	$ 2,361,800	$ 2,286,203
Tier One Leverage Capital to Average Assets	11.75%	11.40%
Tier One Leverage Capital Required to be Well Capitalized	$ 1,107,900	$ 959,477
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	5.00%	5.00%
Excess Tier One Leverage Capital	$ 1,253,900	$ 1,326,726
Tier One Risk Based Common Equity	$ 2,361,800	$ 2,286,203
Tier One Risk Based Common Equity to Risk Weighted Assets	11.91%	11.78%
Tier One Risk Based Common Equity Required to be Well Capitalized	$ 1,407,743	$ 1,206,960
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	$ 954,057	$ 1,079,243
Tier One Risk Based Capital	$ 2,361,800	$ 2,286,203
Tier One Risk Based Capital to Risk Weighted Assets	11.91%	11.78%
Tier One Risk Based Capital Required to be Well Capitalized	$ 1,732,607	$ 1,485,490
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	8.00%	8.00%
Excess Tier One Risk Based Capital	$ 629,193	$ 800,713
Capital	$ 2,978,643	$ 2,889,163
Capital to Risk Weighted Assets	12.69%	12.72%
Capital Required to be Well Capitalized	$ 2,165,759	$ 1,856,862
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Excess Capital	$ 812,884	$ 1,032,301
Pacific Western Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier One Leverage Capital	$ 2,574,561	$ 2,184,097
Tier One Leverage Capital to Average Assets	10.66%	11.91%
Tier One Leverage Capital Required to be Well Capitalized	$ 1,095,656	$ 957,630
Excess Tier One Leverage Capital	1,478,905	1,226,467
Tier One Risk Based Common Equity	$ 2,574,561	$ 2,184,097
Tier One Risk Based Common Equity to Risk Weighted Assets	10.91%	12.31%
Tier One Risk Based Common Equity Required to be Well Capitalized	$ 1,405,299	$ 1,205,541
Excess Common Equity Tier One Capital	1,169,262	978,556
Tier One Risk Based Capital	$ 2,574,561	$ 2,184,097
Tier One Risk Based Capital to Risk Weighted Assets	10.91%	12.31%
Tier One Risk Based Capital Required to be Well Capitalized	$ 1,729,599	$ 1,483,742
Excess Tier One Risk Based Capital	844,962	700,355
Capital	$ 2,742,624	$ 2,358,829
Capital to Risk Weighted Assets	13.75%	15.56%
Capital Required to be Well Capitalized	$ 2,161,999	$ 1,854,678
Excess Capital	$ 580,625	$ 504,151