Goodwill and Other Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Goodwill	$ 2,173,949	$ 2,176,291	$ 1,720,479
Goodwill	2,548,670	2,173,949	2,176,291
CapitalSource Inc. [Member]
Goodwill [Roll Forward]
Goodwill, Subsequent Recognition of Deferred Tax Asset			7,901
Square 1 Financial, Inc. [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period			447,911
Goodwill, Subsequent Recognition of Deferred Tax Asset			$ 1,842
Pacific Western Equipment Finance [Member]
Goodwill [Roll Forward]
Goodwill, Subsequent Recognition of Deferred Tax Asset		$ 500
CU Bancorp [Member] | CU Bancorp [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period	$ 374,721