Acquisitions - Narrative (Details) $ / shares in Units, $ in Thousands				2 Months Ended	3 Months Ended		9 Months Ended
	Jul. 01, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Apr. 01, 2017 USD ($) shares	Dec. 31, 2017 USD ($) $ / shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / shares	Nov. 01, 2017 USD ($)	Mar. 31, 2017 $ / shares
Business Acquisition [Line Items]
Goodwill		$ 493,329		$ 493,329	$ 807,892		$ 493,329
Adjustment to deferred tax assets and the deal consideration						$ 1,100
Grandpoint Capital, Inc.
Business Acquisition [Line Items]
Total assets acquired	$ 3,051,567				3,075,644
Gross loans acquired	2,400,000
Total deposits acquired	$ 2,506,663				2,506,929
Equity issued, ratio	0.4750
Consideration paid	$ 629,000				601,172
Aggregate cash consideration payable	$ 28,100
Company's common stock value (usd per share) | $ / shares	$ 38.15
Goodwill	$ 312,000				$ 312,239
Grandpoint Capital, Inc. | Common Stock
Business Acquisition [Line Items]
Common stock issued as consideration (in shares) | shares	15,758,039
Plaza Bancorp
Business Acquisition [Line Items]
Total assets acquired		1,255,740		1,255,740			1,255,740	$ 1,245,740
Gross loans acquired		1,100,000		1,100,000			1,100,000
Total deposits acquired		$ 1,082,951		$ 1,082,951			$ 1,082,951	$ 1,081,727
Equity issued, ratio		0.2000		0.2000			0.2000
Consideration paid		$ 246,000		$ 245,761
Aggregate cash consideration payable		$ 6,500		$ 6,500			$ 6,500
Company's common stock value (usd per share) | $ / shares		$ 40.40		$ 40.40			$ 40.40
Goodwill		$ 122,629		$ 122,629			$ 122,629
Adjustments to core deposit intangibles, deferred tax assets, loans and other assets and liabilities				1,300
Plaza Bancorp | Common Stock
Business Acquisition [Line Items]
Common stock issued as consideration (in shares) | shares		6,049,373
Heritage Oaks Bancorp
Business Acquisition [Line Items]
Total assets acquired		$ 2,014,822	$ 2,003,635	2,014,822			2,014,822
Gross loans acquired			1,400,000
Total deposits acquired		1,669,550	$ 1,668,085	1,669,550			1,669,550
Equity issued, ratio			0.3471
Consideration paid			$ 467,000				467,439
Aggregate cash consideration payable			3,900
Company's common stock value (usd per share) | $ / shares									$ 38.55
Goodwill		$ 269,553	$ 270,000	$ 269,553			$ 269,553
Heritage Oaks Bancorp | Common Stock
Business Acquisition [Line Items]
Common stock issued as consideration (in shares) | shares			11,959,022