Investment Securities (Tables)

3 Months Ended

Mar. 31, 2020

Investments, Debt and Equity Securities [Abstract]

Amortized Cost and Approximate Fair Values of Investment Securities

The amortized cost and the approximate fair values of investment securities are summarized for the periods indicated:

March 31, 2020	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
U.S. Treasury notes	$	17.1	$	—	$	—		$	17.1
State, county and municipal securities	111.4		0.9		—			112.3
Obligations of U.S. government agencies	202.7		1.0		(0.1		)	203.6
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	2,356.7		60.8		(2.7		)	2,414.8
Private mortgage-backed securities	41.2		—		(1.2		)	40.0
Corporate securities	215.6		0.4		(5.5		)	210.5
Other investments	2.0		—		—			2.0
Total	$	2,946.7	$	63.1	$	(9.5	)	$	3,000.3

March 31, 2020	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	55.2	$	2.3	$	—	$	57.5
Obligations of U.S. government agencies	9.8		—		—		9.8
U.S agency residential mortgage-backed securities & collateralized mortgage obligations	1.2		0.1		—		1.3
Corporate securities	3.9		—		—		3.9
Other investments	0.1		—		—		0.1
Total	$	70.2	$	2.4	$	—	$	72.6

December 31, 2019	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-Sale:
U.S. Treasury notes	$	9.0	$	—	$	—		$	9.0
State, county and municipal securities	80.1		0.8		—			80.9
Obligations of U.S. government agencies	367.5		0.1		(0.8		)	366.8
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	2,303.6		19.6		(6.0		)	2,317.2
Private mortgage-backed securities	47.6		—		(0.4		)	47.2
Corporate securities	134.5		1.2		—			135.7
Other investments	3.2		—		—			3.2
Total	$	2,945.5	$	21.7	$	(7.2	)	$	2,960.0

December 31, 2019	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Held-to-Maturity:
State, county and municipal securities	$	57.3	$	2.1	$	—	$	59.4
Obligations of U.S. government agencies	19.8		—		—		19.8
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	1.2		—		—		1.2
Corporate securities	13.9		0.1		—		14.0
Other investments	0.1		—		—		0.1
Total	$	92.3	$	2.2	$	—	$	94.5

Gross Unrealized Losses and Fair Values of Investment Securities

The following tables show the gross unrealized losses and fair values of available-for-sale 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, 2020 and December 31, 2019. There was no material allowance for credit loss as of March 31, 2020.

	Less than 12 Months					12 Months or More					Total
March 31, 2020	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
Obligations of U.S. government agencies	$	19.9	$	(0.1	)	$	—	$	—		$	19.9	$	(0.1	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	143.5		(1.7		)	133.6		(1.0		)	277.1		(2.7		)
Private mortgage-backed securities	—		—			40.0		(1.2		)	40.0		(1.2		)
Corporate securities	119.9		(4.9		)	39.9		(0.6		)	159.8		(5.5		)
Total	$	283.3	$	(6.7	)	$	213.5	$	(2.8	)	$	496.8	$	(9.5	)

	Less than 12 Months					12 Months or More					Total
December 31, 2019	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Available-for-Sale:
Obligations of U.S. government agencies	$	185.3	$	(0.8	)	$	—	$	—		$	185.3	$	(0.8	)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations	740.1		(4.6		)	155.9		(1.4		)	896.0		(6.0		)
Private mortgage-backed securities	—		—			46.6		(0.4		)	46.6		(0.4		)
Total	$	925.4	$	(5.4	)	$	202.5	$	(1.8	)	$	1,127.9	$	(7.2	)

Maturities of Investment Securities

Maturities of investment securities at March 31, 2020 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, 2020	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Within one year	$	614.3	$	608.0	$	19.7	$	19.8
After one year but within five years	1,416.2		1,422.1		27.4		27.8
After five years but within ten years	421.2		523.5		21.3		23.2
After ten years	495.0		446.7		1.8		1.8
Total	$	2,946.7	$	3,000.3	$	70.2	$	72.6