INVESTMENT SECURITIES

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



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 investments	354		1		—			355
Total	$	600,665	$	12,854	$	(5,969	)	$	607,550

Gross gains of $156 and gross losses of $19 were realized on the disposition of available-for-sale securities in 2015.



December 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale
Obligations of U.S. government agencies	$	725,408	$	895	$	(5,370	)	$	720,933
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	982,764		11,526		(3,624		)	990,666
Private mortgage-backed securities	322		5		(2		)	325
Total	$	1,708,494	$	12,426	$	(8,996	)	$	1,711,924



December 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to Maturity
State, county and municipal securities	$	188,941	$	5,949	$	(386	)	$	194,504
Corporate securities	32,565		54		(75		)	32,544
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	353,176		5,563		(1,758		)	356,981
Other securities	504		—		—			504
Total	$	575,186	$	11,566	$	(2,219	)	$	584,533

Gross gains of $274 and $49 were realized on the disposition of available-for-sale securities in 2014 and 2013, respectively. Gross losses of $213 and $48 were realized on the disposition of available-for-sale securities in 2014 and 2013, respectively.

As of December 31, 2015, the Company had general obligation securities with amortized costs of $140,458 included in state, county and municipal securities, of which $78,337 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, 2015 and 2014.



	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	)



	Less than 12 Months					12 Months or More					Total
December 31, 2014	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale
Obligations of U.S. government agencies	$	135,888	$	(702	)	$	309,283	$	(4,668	)	$	445,171	$	(5,370	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	219,214		(887		)	151,380		(2,737		)	370,594		(3,624		)
Private mortgage-backed securities	—		—			90		(2		)	90		(2		)
Total	$	355,102	$	(1,589	)	$	460,753	$	(7,407	)	$	815,855	$	(8,996	)



	Less than 12 Months					12 Months or More					Total
December 31, 2014	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity
State, county and municipal securities	$	7,979	$	(13	)	$	20,097	$	(373	)	$	28,076	$	(386	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	61,201		(1,758		)	—		—			$	61,201	$	(1,758	)
Corporate securities	14,755		(75		)	—		—			14,755		(75		)
Total	$	83,935	$	(1,846	)	$	20,097	$	(373	)	$	104,032	$	(2,219	)

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, 2015, the Company had 198 individual investment securities that were in an unrealized loss position. As of December 31, 2014, the Company had 154 individual investment securities that were in an unrealized loss position. Unrealized losses as of December 31, 2015 and 2014 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, 2015, 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 2015, 2014 or 2013.

Maturities of investment securities at December 31, 2015 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, 2015	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	329,638	$	331,620	$	100,695	$	101,885
After one year but within five years	1,018,573		1,022,351		320,310		322,281
After five years but within ten years	46,874		47,117		134,606		137,304
After ten years	55,311		55,752		45,054		46,080
Total	$	1,450,396	$	1,456,840	$	600,665	$	607,550

At December 31, 2015, the Company had investment securities callable within one year with amortized costs and estimated fair values of $98,885 and $98,664, 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, 2015, 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, 2015 and 2014, the Company had variable rate mortgage-backed securities with amortized costs of $68,440 and $40,666, respectively, classified as available-for-sale in the table above.

There are no significant concentrations of investments at December 31, 2015, (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 $1,325,078 and $1,351,787 at December 31, 2015 and 2014, respectively, were pledged to secure public deposits and securities sold under repurchase agreements. The approximate fair value of securities pledged at December 31, 2015 and 2014 was $1,328,571 and $1,356,341, 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.