Goodwill and Core Deposit Intangibles - Goodwill Roll Forward (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Balance, beginning of year		$ 493,329,000	$ 102,490,000	$ 50,832,000
Goodwill acquired during the year		313,043,000	390,839,000	51,658,000
Purchase Accounting Adjustments		2,354,000	0	0
Impairment losses	$ 0	0	0	0
Balance, end of year	808,726,000	808,726,000	493,329,000	102,490,000
Accumulated impairment losses at end of year	$ 0	$ 0	$ 0	$ 0