Investment Securities (Tables)

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Amortized Cost and Approximate Fair Values of Investment Securities

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

March 31, 2018	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	562.8		—		(13.0		)	549.8
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1,578.2		1.9		(30.8		)	1,549.3
Private mortgage-backed securities	84.7		—		(1.5		)	83.2
Corporate securities	93.1		0.1		(1.0		)	92.2
Other investments	3.0		—		—			3.0
Total	$	2,325.0	$	2.0	$	(46.3	)	$	2,280.7

March 31, 2018	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	168.3	$	1.8	$	(1.3	)	$	168.8
Obligations of U.S. government agencies	19.8		—		(0.3		)	19.5
U.S agency residential mortgage-backed securities & collateralized mortgage obligations	217.6		7.7		(14.1		)	211.2
Corporate securities	56.5		0.1		(0.7		)	55.9
Other investments	0.1		—		—			0.1
Total	$	462.3	$	9.6	$	(16.4	)	$	455.5

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
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
Corporate securities	61.6		0.1		(0.3		)	61.4
Other investments	0.2		—		—			0.2
Total	$	484.5	$	11.5	$	(12.7	)	$	483.3

Gross Unrealized Losses and Fair Values of Investment Securities

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 March 31, 2018 and December 31, 2017:

	Less than 12 Months					12 Months or More					Total
March 31, 2018	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
Obligations of U.S. government agencies	$	310.1	$	(7.0	)	$	239.7	$	(6.0	)	$	549.8	$	(13.0	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	954.2		(17.5		)	408.4		(13.3		)	1,362.6		(30.8		)
Private mortgage-backed securities	74.7		(1.5		)	—		—			74.7		(1.5		)
Corporate securities	60.8		(1.0		)	—		—			60.8		(1.0		)
Total	$	1,399.8	$	(27.0	)	$	648.1	$	(19.3	)	$	2,047.9	$	(46.3	)

	Less than 12 Months					12 Months or More					Total
March 31, 2018	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Held-to-Maturity:
State, county and municipal securities	$	74.0	$	(1.0	)	$	11.7	$	(0.3	)	$	85.7	$	(1.3	)
Obligations of U.S. government agencies	9.6		(0.1		)	9.9		(0.2		)	19.5		(0.3		)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	85.6		(11.0		)	47.2		(3.1		)	132.8		(14.1		)
Corporate securities	44.8		(0.6		)	4.9		(0.1		)	49.7		(0.7		)
Total	$	214.0	$	(12.7	)	$	73.7	$	(3.7	)	$	287.7	$	(16.4	)

	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	)
Obligations of U.S. government agencies	9.7		—			9.9		(0.2		)	19.6		(0.2		)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	76.4		(9.1		)	60.5		(2.5		)	136.9		(11.6		)
Corporate securities	41.2		(0.2		)	5.0		(0.1		)	46.2		(0.3		)
Total	$	180.6	$	(9.7	)	$	87.7	$	(3.0	)	$	268.3	$	(12.7	)

Maturities of Investment Securities

Maturities of investment securities at March 31, 2018 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
March 31, 2018	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	433.0	$	426.3	$	61.4	$	61.2
After one year but within five years	1,445.7		1,417.8		247.2		241.6
After five years but within ten years	278.8		272.2		121.0		120.8
After ten years	167.5		164.4		32.7		31.9
Total	$	2,325.0	$	2,280.7	$	462.3	$	455.5