Consideration Transferred, Identifiable Net Assets Acquired and Goodwill Recognized (Details) - USD ($)					12 Months Ended
		Feb. 28, 2018	Jan. 31, 2018	Apr. 30, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair value of consideration transferred
Effective settlement of pre-existing receivable					$ 10,054,000	$ 0	$ 0
Liabilities assumed
Goodwill recognized					$ 111,775,000	$ 30,758,000	$ 6,415,000
First Security Bank
Fair value of consideration transferred
Fair value of Company shares issued, net of equity issuance costs		$ 181,043,000
Cash consideration for outstanding shares		0
Effective settlement of pre-existing receivable		0
Total fair value of consideration transferred		181,043,000
Identifiable assets acquired
Cash and cash equivalents		24,397,000
Debt securities		271,865,000
Loans receivable		627,767,000
Core deposit intangible	[1]	31,053,000
Accrued income and other assets		78,274,000
Total identifiable assets acquired		1,033,356,000
Liabilities assumed
Deposits		877,586,000
Borrowings	[2]	36,880,000
Accrued expenses and other liabilities		14,175,000
Total liabilities assumed		928,641,000
Total identifiable net assets		104,715,000
Goodwill recognized		$ 76,328,000
Core Deposit Intangible, Weighted Average Useful Life		10 years
Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Subordinated Debentures		$ 7,903,000
Collegiate Peaks Bank
Fair value of consideration transferred
Fair value of Company shares issued, net of equity issuance costs			$ 69,764,000
Cash consideration for outstanding shares			16,265,000
Effective settlement of pre-existing receivable			10,054,000
Total fair value of consideration transferred			96,083,000
Identifiable assets acquired
Cash and cash equivalents			93,136,000
Debt securities			42,177,000
Loans receivable			354,252,000
Core deposit intangible	[1]		10,275,000
Accrued income and other assets			15,911,000
Total identifiable assets acquired			515,751,000
Liabilities assumed
Deposits			437,171,000
Borrowings			12,509,000
Accrued expenses and other liabilities			5,435,000
Total liabilities assumed			455,115,000
Total identifiable net assets			60,636,000
Goodwill recognized			$ 35,447,000
Core Deposit Intangible, Weighted Average Useful Life			10 years
The Foothills Bank
Fair value of consideration transferred
Fair value of Company shares issued, net of equity issuance costs				$ 46,673,000
Cash consideration for outstanding shares				17,342,000
Effective settlement of pre-existing receivable				0
Total fair value of consideration transferred				64,015,000
Identifiable assets acquired
Cash and cash equivalents				13,251,000
Debt securities				25,420,000
Loans receivable				292,529,000
Core deposit intangible	[1]			4,331,000
Accrued income and other assets				19,699,000
Total identifiable assets acquired				355,230,000
Liabilities assumed
Deposits				296,760,000
Borrowings				22,800,000
Accrued expenses and other liabilities				2,264,000
Total liabilities assumed				321,824,000
Total identifiable net assets				33,406,000
Goodwill recognized				$ 30,609,000
Core Deposit Intangible, Weighted Average Useful Life				10 years

[1] The core deposit intangible for each acquisition was determined to have an estimated life of 10 years.

[2] Borrowings assumed with the FSB acquisition include Tier 2 subordinated debentures of $7,903,000.