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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Sep. 30, 2018
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Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses | $ (1,613) |
Less than 12 months, Unrealized Gross Losses |  | $ (29,854)
Less than 12 months, Fair Value | 294,394 |
Less than 12 months, Fair Value |  | 1,124,253
12 months or more, Unrealized Gross Losses | (27,807) |
12 months or more, Unrealized Gross Losses |  | (63,328)
12 months or more, Fair Value | 1,800,801 |
12 months or more, Fair Value |  | 1,284,504
Total Unrealized Gross Losses | (29,420) |
Total Unrealized Gross Losses |  | (93,182)
Total, Fair Value | 2,095,195 |
Total, Fair Value |  | 2,408,757
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses | (574) | (929)
Less than 12 months, Fair Value | 49,521 | 49,072
12 months or more, Unrealized Gross Losses | (263) | (14)
12 months or more, Fair Value | 49,737 | 24,988
Total Unrealized Gross Losses | (837) | (943)
Total, Fair Value | 99,258 | 74,060
Municipal bonds due |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses |  | (24)
Less than 12 months, Fair Value |  | 1,374
12 months or more, Unrealized Gross Losses |  | 0
12 months or more, Fair Value |  | 0
Total Unrealized Gross Losses |  | (24)
Total, Fair Value |  | 1,374
U.S. government and agency securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses | (621) | (141)
Less than 12 months, Fair Value | 84,101 | 37,565
12 months or more, Unrealized Gross Losses | (1,619) | (1,645)
12 months or more, Fair Value | 129,356 | 76,499
Total Unrealized Gross Losses | (2,240) | (1,786)
Total, Fair Value | 213,457 | 114,064
Equity securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses |  | (12)
Less than 12 months, Fair Value |  | 488
12 months or more, Unrealized Gross Losses |  | 0
12 months or more, Fair Value |  | 0
Total Unrealized Gross Losses |  | (12)
Total, Fair Value |  | 488
Mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 months, Unrealized Gross Losses | (418) | (28,748)
Less than 12 months, Fair Value | 160,772 | 1,035,754
12 months or more, Unrealized Gross Losses | (25,925) | (61,669)
12 months or more, Fair Value | 1,621,708 | 1,183,017
Total Unrealized Gross Losses | (26,343) | (90,417)
Total, Fair Value | $ 1,782,480 | $ 2,218,771

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