Investment in Securities - Credit Quality Indicators for Amortized Cost of Held-to-maturity Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	$ 8,576,192	$ 8,631,261
AAA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	136,474
AA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	441,110
A
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	3,711,611
BBB
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	3,978,891
BB and below
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	308,106
U.S. states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	155,151	165,109
U.S. states and political subdivisions | AAA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	54,145
U.S. states and political subdivisions | AA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	71,923
U.S. states and political subdivisions | A
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	23,814
U.S. states and political subdivisions | BBB
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	0
U.S. states and political subdivisions | BB and below
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	5,269
Foreign governments
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	3,894	3,907
Foreign governments | AAA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	0
Foreign governments | AA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	2,852
Foreign governments | A
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	1,042
Foreign governments | BBB
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	0
Foreign governments | BB and below
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	0
Corporate debt securities
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	8,029,559	8,099,098
Corporate debt securities | AAA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	1,942
Corporate debt securities | AA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	366,335
Corporate debt securities | A
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	3,517,245
Corporate debt securities | BBB
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	3,962,838
Corporate debt securities | BB and below
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	181,199
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	179,087	237,516
Residential mortgage-backed securities | AAA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	80,387
Residential mortgage-backed securities | AA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	0
Residential mortgage-backed securities | A
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	4,745
Residential mortgage-backed securities | BBB
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	0
Residential mortgage-backed securities | BB and below
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	93,955
Collateralized debt securities
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	208,501	$ 125,631
Collateralized debt securities | AAA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	0
Collateralized debt securities | AA
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	0
Collateralized debt securities | A
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	164,765
Collateralized debt securities | BBB
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	16,053
Collateralized debt securities | BB and below
Schedule of Held-to-maturity Securities [Line Items]
Carrying value of bonds held-to-maturity	$ 27,683