INVESTMENT SECURITIES

The amortized cost and approximate fair values of investment securities are summarized as follows:



December 31, 2016	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale
U.S. Treasury notes	$	3,608	$	5	$	(1	)	$	3,612
Obligations of U.S. government agencies	397,411		343		(6,457		)	391,297
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,220,890		6,412		(13,601		)	1,213,701
Private mortgage-backed securities	116		1		(1		)	116
Other investments	2,951		21		—			2,972
Total	$	1,624,976	$	6,782	$	(20,060	)	$	1,611,698



December 31, 2016	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to Maturity
State, county and municipal securities	$	160,192	$	2,723	$	(542	)	$	162,373
Corporate securities	53,032		139		(211		)	52,960
Obligations of U.S. government agencies	19,737		—		(162		)	19,575
U.S agency residential mortgage-backed securities & collateralized mortgage obligations	279,578		7,804		(9,249		)	278,133
Other investments	231		1		—			232
Total	$	512,770	$	10,667	$	(10,164	)	$	513,273

Gross gains of $423 and gross losses of $93 were realized on the disposition of available-for-sale securities in 2016.



December 31, 2015	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale
U.S. Treasury notes	$	3,912	$	3	$	(4	)	$	3,911
Obligations of U.S. government agencies	521,079		712		(1,610		)	520,181
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	921,699		9,448		(2,101		)	929,046
Private mortgage-backed securities	156		1		(1		)	156
Other investments	3,550		5		(9		)	3,546
Total	$	1,450,396	$	10,169	$	(3,725	)	$	1,456,840



December 31, 2015	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to Maturity
State, county and municipal securities	$	173,785	$	5,103	$	(227	)	$	178,661
Corporate securities	50,046		64		(220		)	49,890
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	376,480		7,686		(5,522		)	378,644
Other securities	354		1		—			355
Total	$	600,665	$	12,854	$	(5,969	)	$	607,550

Gross gains of $156 and $274 were realized on the disposition of available-for-sale securities in 2015 and 2014, respectively. Gross losses of $19 and $213 were realized on the disposition of available-for-sale securities in 2015 and 2014, respectively.

As of December 31, 2016, the Company had general obligation securities with amortized costs of $131,020 included in state, county and municipal securities, of which $78,725 were issued by political subdivisions or agencies within the states of Montana, Wyoming and South Dakota.

On October 30, 2015, the Company transferred available-for-sale U.S. agency residential mortgage-backed securities and collateralized mortgage obligations with amortized costs and fair values of $100,343 and $100,140, respectively, into the held-to-maturity category. Unrealized net losses of $203 included in accumulated other comprehensive income at the time of the transfer are being amortized to yield over the remaining expected lives of the transferred securities of 4.0 years.

On June 27, 2014, the Company transferred available-for-sale U.S. agency residential mortgage-backed securities and collateralized mortgage obligations with amortized costs and fair values of $396,640 and $388,808, respectively, into the held-to-maturity category. Unrealized net losses of $7,832 at the time of the transfer are being amortized to yield over the remaining expected lives of the transferred securities of 4.3 years.

The following tables show the gross unrealized losses and fair values of investment securities, aggregated by investment category, and the length of time individual investment securities have been in a continuous unrealized loss position, as of December 31, 2016 and 2015.



	Less than 12 Months					12 Months or More					Total
December 31, 2016	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale
U.S. Treasury notes	$	598	$	(1	)	$	—	$	—		$	598	$	(1	)
Obligations of U.S. government agencies	316,511		(6,457		)	—		—			316,511		(6,457		)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	746,265		(13,102		)	15,801		(499		)	762,066		(13,601		)
Private mortgage-backed securities	—		—			48		(1		)	48		(1		)
Other investments	—		—			—		—			—		—
Total	$	1,063,374	$	(19,560	)	$	15,849	$	(500	)	$	1,079,223	$	(20,060	)



	Less than 12 Months					12 Months or More					Total
December 31, 2016	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity
State, county and municipal securities	$	42,518	$	(533	)	$	2,831	$	(9	)	$	45,349	$	(542	)
Corporate securities	32,474		(211		)	—		—			32,474		(211		)
Obligations of U.S. government agencies	19,575		(162		)	—		—			19,575		(162		)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	108,857		(7,973		)	19,986		(1,276		)	128,843		(9,249		)
Total	$	203,424	$	(8,879	)	$	22,817	$	(1,285	)	$	226,241	$	(10,164	)



	Less than 12 Months					12 Months or More					Total
December 31, 2015	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale
U.S. Treasury notes	$	2,092	$	(4	)	$	—	$	—		$	2,092	$	(4	)
Obligations of U.S. government agencies	209,631		(1,077		)	54,619		(533		)	264,250		(1,610		)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	343,875		(1,577		)	28,010		(524		)	371,885		(2,101		)
Private mortgage-backed securities	—		—			61		(1		)	61		(1		)
Other investments	1,225		(9		)	—		—			1,225		(9		)
Total	$	556,823	$	(2,667	)	$	82,690	$	(1,058	)	$	639,513	$	(3,725	)



	Less than 12 Months					12 Months or More					Total
December 31, 2015	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity
State, county and municipal securities	$	10,182	$	(39	)	$	9,476	$	(188	)	$	19,658	$	(227	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	87,033		(4,390		)	69,539		(1,132		)	$	156,572	$	(5,522	)
Corporate securities	31,135		(220		)	—		—			31,135		(220		)
Total	$	128,350	$	(4,649	)	$	79,015	$	(1,320	)	$	207,365	$	(5,969	)

The investment portfolio is evaluated quarterly for other-than-temporary declines in the market value of each individual investment security. Consideration is given to the length of time and the extent to which the fair value has been less than cost; the financial condition and near term prospects of the issuer; and, the intent and ability of the Company to retain its investment in the issuer for a period of time sufficient to allow for any anticipated recovery in fair value. As of December 31, 2016, the Company had 396 individual investment securities that were in an unrealized loss position. As of December 31, 2015, the Company had 198 individual investment securities that were in an unrealized loss position. Unrealized losses as of December 31, 2016 and 2015 related primarily to fluctuations in the current interest rates. The fair value of these investment securities is expected to recover as the securities approach their maturity or repricing date or if market yields for such investments decline. As of December 31, 2016, the Company had the intent and ability to hold these investment securities for a period of time sufficient to allow for an anticipated recovery. Furthermore, the Company does not have the intent to sell any of the available-for-sale securities in the above table and it is more likely than not that the Company will not have to sell any securities before a recovery in cost. No impairment losses were recorded during 2016, 2015 or 2014.

Maturities of investment securities at December 31, 2016 are shown below. Maturities of mortgage-backed securities have been adjusted to reflect shorter maturities based upon estimated prepayments of principal. All other investment securities maturities are shown at contractual maturity dates.



	Available-for-Sale				Held-to-Maturity
December 31, 2016	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	343,675	$	341,720	$	89,581	$	90,105
After one year but within five years	1,109,151		1,100,551		285,138		283,668
After five years but within ten years	135,520		133,012		111,437		112,852
After ten years	36,630		36,415		26,614		26,648
Total	$	1,624,976	$	1,611,698	$	512,770	$	513,273

At December 31, 2016, the Company had investment securities callable within one year with amortized costs and estimated fair values of $140,083 and $137,562, respectively. These investment securities are primarily classified as available-for-sale and included in the after one year but within five years category in the table above. At December 31, 2016, the Company had no callable structured notes.

Maturities of securities do not reflect rate repricing opportunities present in adjustable rate mortgage-backed securities. At December 31, 2016 and 2015, the Company had variable rate mortgage-backed securities with amortized costs of $66,224 and $68,440, respectively, classified as available-for-sale in the table above.

There are no significant concentrations of investments at December 31, 2016, (greater than 10 percent of stockholders’ equity) in any individual security issuer, except for U.S. government or agency-backed securities.

Investment securities with amortized cost of $1,400,060 and $1,325,078 at December 31, 2016 and 2015, respectively, were pledged to secure public deposits and securities sold under repurchase agreements. The approximate fair value of securities pledged at December 31, 2016 and 2015 was $1,388,200 and $1,328,571, respectively. All securities sold under repurchase agreements are with customers and mature on the next banking day. The Company retains possession of the underlying securities sold under repurchase agreements.