INVESTMENT SECURITIES

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



December 31, 2017	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale
U.S. Treasury notes	$	3.2	$	—	$	—		$	3.2
Obligations of U.S. government agencies	569.5		—		(8.0		)	561.5
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,474.1		3.8		(15.4		)	1,462.5
Private mortgage-backed securities	91.5		—		(0.8		)	90.7
Corporate Securities	88.0		0.1		(0.3		)	87.8
Other investments	3.0		—		—			3.0
Total	$	2,229.3	$	3.9	$	(24.5	)	$	2,208.7



December 31, 2017	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to Maturity
State, county and municipal securities	$	172.4	$	2.6	$	(0.6	)	$	174.4
Corporate securities	61.6		0.1		(0.3		)	61.4
Obligations of U.S. government agencies	19.8		—		(0.2		)	19.6
U.S agency residential mortgage-backed securities & collateralized mortgage obligations	230.5		8.8		(11.6		)	227.7
Other investments	0.2		—		—			0.2
Total	$	484.5	$	11.5	$	(12.7	)	$	483.3



December 31, 2016	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale
U.S. Treasury notes	$	3.6	$	—	$	—		$	3.6
Obligations of U.S. government agencies	397.4		0.3		(6.4		)	391.3
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,220.9		6.4		(13.6		)	1,213.7
Private mortgage-backed securities	0.1		—		—			0.1
Other investments	3.0		—		—			3.0
Total	$	1,625.0	$	6.7	$	(20.0	)	$	1,611.7



December 31, 2016	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to Maturity
State, county and municipal securities	$	160.2	$	2.7	$	(0.5	)	$	162.4
Corporate securities	53.0		0.2		(0.2		)	53.0
Obligations of U.S. government agencies	19.8		—		(0.2		)	19.6
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	279.6		7.8		(9.2		)	278.2
Other investments	0.2		—		—			0.2
Total	$	512.8	$	10.7	$	(10.1	)	$	513.4

Gross gains of $1.1 million and gross losses of $0.4 million were realized on the disposition of available-for-sale securities in 2017.

Gross gains of $0.4 million and $0.2 million were realized on the disposition of available-for-sale securities in 2016 and 2015, respectively. Gross losses of $0.1 million and $0.02 million were realized on the disposition of available-for-sale securities in 2016 and 2015, respectively.

As of December 31, 2017, the Company had general obligation securities with amortized costs of $133.5 million included in state, county and municipal securities, of which $108.2 million were issued by political subdivisions or agencies within the states of Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming.

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 $100.3 million and $100.1 million, respectively, into the held-to-maturity category. Unrealized net losses of $0.2 million 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.

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, 2017 and 2016.



	Less than 12 Months					12 Months or More					Total
December 31, 2017	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale
Obligations of U.S. government agencies	$	284.9	$	(3.4	)	$	266.1	$	(4.6	)	$	551.0	$	(8.0	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	670.1		(6.2		)	439.2		(9.2		)	1,109.3		(15.4		)
Private mortgage-backed securities	74.0		(0.8		)	—		—			74.0		(0.8		)
Corporate securities	51.3		(0.3		)	—		—			51.3		(0.3		)
Total	$	1,080.3	$	(10.7	)	$	705.3	$	(13.8	)	$	1,785.6	$	(24.5	)



	Less than 12 Months					12 Months or More					Total
December 31, 2017	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity
State, county and municipal securities	$	53.3	$	(0.4	)	$	12.3	$	(0.2	)	$	65.6	$	(0.6	)
Corporate securities	41.2		(0.2		)	5.0		(0.1		)	46.2		(0.3		)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	76.4		(9.1		)	60.5		(2.5		)	136.9		(11.6		)
Obligations of U.S. government agencies	9.7		—			9.9		(0.2		)	19.6		(0.2		)
Total	$	180.6	$	(9.7	)	$	87.7	$	(3.0	)	$	268.3	$	(12.7	)



	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	$	0.6	$	—		$	—	$	—		$	0.6	$	—
Obligations of U.S. government agencies	316.5		(6.4		)	—		—			316.5		(6.4		)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	746.3		(13.1		)	15.8		(0.5		)	762.1		(13.6		)
Total	$	1,063.4	$	(19.5	)	$	15.8	$	(0.5	)	$	1,079.2	$	(20.0	)



	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	$	42.5	$	(0.5	)	$	2.8	$	—		$	45.3	$	(0.5	)
Corporate securities	32.5		(0.2		)	—		—			$	32.5	$	(0.2	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	$	108.8	$	(7.9	)	$	20.0	$	(1.3	)	$	128.8	$	(9.2	)
Obligations of U.S. government agencies	$	19.6	$	(0.2	)	$	—	$	—		$	19.6	$	(0.2	)
Total	$	203.4	$	(8.8	)	$	22.8	$	(1.3	)	$	226.2	$	(10.1	)

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. The Company had 581 and 396 individual investment securities that were in an unrealized loss position as of December 31, 2017 and 2016, respectively. Unrealized losses as of December 31, 2017 and 2016 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, 2017, 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 2017, 2016 or 2015.

Maturities of investment securities at December 31, 2017 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, 2017	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	450.7	$	447.5	$	76.1	$	76.7
After one year but within five years	1,422.7		1,409.3		266.2		263.1
After five years but within ten years	277.6		274.2		111.7		113.3
After ten years	78.3		77.7		30.5		30.2
Total	$	2,229.3	$	2,208.7	$	484.5	$	483.3

At December 31, 2017, the Company had investment securities callable within one year with amortized costs and estimated fair values of $133.2 million and $132.9 million, 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, 2017, 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, 2017 and 2016, the Company had variable rate mortgage-backed securities with amortized costs of $247.9 million and $66.2 million, respectively, classified as available-for-sale in the table above.

There are no significant concentrations of investments at December 31, 2017, (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 $2,087.7 million and $1,400.1 million at December 31, 2017 and 2016, respectively, were pledged to secure public deposits and securities sold under repurchase agreements. The approximate fair value of securities pledged at December 31, 2017 and 2016 was $2,062.6 million and $1,388.2 million, 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.