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Goodwill and Core Deposit Intangibles - Goodwill Roll Forward (Details) - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Goodwill [Roll Forward]      
Balance, beginning of year   $ 102,490,000 $ 50,832,000
Goodwill acquired during the year   390,839,000 51,658,000
Impairment losses $ 0 0 0
Balance, end of year 493,329,000 493,329,000 102,490,000
Accumulated impairment losses at end of year $ 0 $ 0 $ 0