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Investment in Securities - Investments in Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Investment [Line Items]    
Bonds held-to-maturity securities, amortized cost $ 8,208,129 $ 8,211,449
Bonds held-to-maturity securities, gross unrealized gains 159,674 72,403
Bonds held-to-maturity securities, gross unrealized (losses) (44,203) (153,768)
Bonds held-to-maturity securities, fair value 8,323,600 8,130,084
Bonds available-for-sale securities, amortized cost 6,476,284 6,261,621
Bonds available-for-sale securities, gross unrealized gains 143,895 61,286
Bonds available-for-sale securities, gross unrealized (losses) (35,786) (107,344)
Bonds available-for-sale securities, fair value 6,584,393 6,215,563
Bonds investments in securities, cost or amortized cost 14,684,413 14,473,070
Bonds investments in securities, gross unrealized gains 303,569 133,689
Bonds investments in securities, gross unrealized (losses) (79,989) (261,112)
Bonds investments in securities, fair value 14,907,993 14,345,647
U.S. States and Political Subdivisions    
Investment [Line Items]    
Bonds held-to-maturity securities, amortized cost 222,183 245,360
Bonds held-to-maturity securities, gross unrealized gains 6,273 5,840
Bonds held-to-maturity securities, gross unrealized (losses) (54) (301)
Bonds held-to-maturity securities, fair value 228,402 250,899
Bonds available-for-sale securities, amortized cost 922,081 848,228
Bonds available-for-sale securities, gross unrealized gains 28,343 16,827
Bonds available-for-sale securities, gross unrealized (losses) (391) (3,025)
Bonds available-for-sale securities, fair value 950,033 862,030
U.S. Treasury and Government    
Investment [Line Items]    
Bonds available-for-sale securities, amortized cost 28,302 28,304
Bonds available-for-sale securities, gross unrealized gains 371 338
Bonds available-for-sale securities, gross unrealized (losses) (147) (243)
Bonds available-for-sale securities, fair value 28,526 28,399
Foreign Governments    
Investment [Line Items]    
Bonds held-to-maturity securities, amortized cost 3,948 3,961
Bonds held-to-maturity securities, gross unrealized gains 468 469
Bonds held-to-maturity securities, gross unrealized (losses) 0 0
Bonds held-to-maturity securities, fair value 4,416 4,430
Bonds available-for-sale securities, amortized cost 5,000 5,000
Bonds available-for-sale securities, gross unrealized gains 1,290 1,210
Bonds available-for-sale securities, gross unrealized (losses) 0 0
Bonds available-for-sale securities, fair value 6,290 6,210
Corporate Debt Securities    
Investment [Line Items]    
Bonds held-to-maturity securities, amortized cost 7,657,374 7,640,891
Bonds held-to-maturity securities, gross unrealized gains 144,853 58,772
Bonds held-to-maturity securities, gross unrealized (losses) (42,537) (150,834)
Bonds held-to-maturity securities, fair value 7,759,690 7,548,829
Bonds available-for-sale securities, amortized cost 5,486,787 5,345,579
Bonds available-for-sale securities, gross unrealized gains 112,670 41,812
Bonds available-for-sale securities, gross unrealized (losses) (34,916) (103,573)
Bonds available-for-sale securities, fair value 5,564,541 5,283,818
Residential Mortgage-Backed Securities    
Investment [Line Items]    
Bonds held-to-maturity securities, amortized cost 216,260 315,306
Bonds held-to-maturity securities, gross unrealized gains 6,884 7,237
Bonds held-to-maturity securities, gross unrealized (losses) (1,350) (2,633)
Bonds held-to-maturity securities, fair value 221,794 319,910
Bonds available-for-sale securities, amortized cost 24,292 31,735
Bonds available-for-sale securities, gross unrealized gains 503 424
Bonds available-for-sale securities, gross unrealized (losses) (326) (497)
Bonds available-for-sale securities, fair value 24,469 31,662
Collateralized Debt Securities    
Investment [Line Items]    
Bonds held-to-maturity securities, amortized cost 108,215 5,214
Bonds held-to-maturity securities, gross unrealized gains 1,194 71
Bonds held-to-maturity securities, gross unrealized (losses) (262) 0
Bonds held-to-maturity securities, fair value 109,147 5,285
Bonds available-for-sale securities, amortized cost 9,822 2,775
Bonds available-for-sale securities, gross unrealized gains 718 675
Bonds available-for-sale securities, gross unrealized (losses) (6) (6)
Bonds available-for-sale securities, fair value 10,534 3,444
Other Debt Securities    
Investment [Line Items]    
Bonds held-to-maturity securities, amortized cost 149 717
Bonds held-to-maturity securities, gross unrealized gains 2 14
Bonds held-to-maturity securities, gross unrealized (losses) 0 0
Bonds held-to-maturity securities, fair value $ 151 $ 731