Acquisitions - Narrative (Details) $ / shares in Units, $ in Thousands			3 Months Ended	5 Months Ended	12 Months Ended
	Nov. 01, 2017 USD ($) $ / shares shares	Apr. 01, 2017 USD ($) shares	Sep. 30, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 $ / shares
Business Acquisition [Line Items]
Company's common stock value (usd per share) | $ / shares							$ 38.55
Goodwill				$ 493,785	$ 493,785	$ 493,329
Adjustment to deferred tax assets and the deal consideration			$ 1,100
Plaza Bancorp
Business Acquisition [Line Items]
Total assets acquired	$ 1,245,740			1,256,290	1,256,290
Gross loans acquired	1,100,000
Total deposits acquired	$ 1,081,727			1,082,951	1,082,951
Equity issued, ratio	0.2000
Consideration paid	$ 246,000			245,761
Aggregate cash consideration payable	$ 6,500
Common stock issued as consideration (in shares) | shares	6,049,373
Company's common stock value (usd per share) | $ / shares	$ 40.40
Goodwill	$ 122,000			121,908	121,908
Heritage Oaks Bancorp
Business Acquisition [Line Items]
Total assets acquired		$ 2,005,637		2,015,822	2,015,822
Gross loans acquired		1,400,000
Total deposits acquired		$ 1,668,079		1,669,550	1,669,550
Equity issued, ratio		0.3471
Consideration paid		$ 468,000			468,131
Aggregate cash consideration payable		3,900
Goodwill		$ 269,000		$ 268,954	$ 268,954
Heritage Oaks Bancorp | Common Stock
Business Acquisition [Line Items]
Common stock issued as consideration (in shares) | shares		11,959,022