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