Goodwill and Core Deposit Intangibles - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 01, 2017	Apr. 01, 2017
Goodwill [Line Items]
Goodwill	$ 493,329,000	$ 493,329,000	$ 102,490,000	$ 50,832,000
Goodwill acquired during the year		390,839,000	51,658,000
Impairment losses	0	0	0
Net CDI	43,014,000	43,014,000	9,451,000	7,170,000
Additions due to acquisitions		39,707,000	$ 4,320,000	$ 2,906,000
Estimated aggregate amortization expense
2018	8,400,000	8,400,000
2019	7,300,000	7,300,000
2020	6,500,000	6,500,000
2021	5,400,000	5,400,000
2022	4,500,000	4,500,000
Plaza Bancorp
Goodwill [Line Items]
Goodwill	121,885,000	121,885,000			$ 122,000,000
Additions due to acquisitions		11,600,000
Heritage Oaks Bank
Goodwill [Line Items]
Goodwill	$ 268,954,000	268,954,000				$ 269,000,000
Additions due to acquisitions		$ 28,100,000