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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Sep. 30, 2017
Amortized Cost, Available-for-sale securities    
Amortized Cost $ 1,238,197 $ 1,256,587
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains 12,786 13,829
Gross unrealized loss (5,128) (4,207)
Fair Value, Available-for-sale securities    
Available-for-sale securities, at fair value $ 1,245,855 $ 1,266,209
Yield, Available-for-sale securities    
Yield 2.98% 2.86%
Amortized Cost, Held-to-maturity securities    
Held-to-maturity securities, at amortized cost $ 1,765,886 $ 1,646,856
Gross Unrealized Gains / Losses, Held-to-maturity securities    
Held-to-maturity securities, Gross unrealized gains 6,223 7,143
Held-to-maturity securities ,Gross unrealized losses (18,767) (18,086)
Fair Value, Held-to-maturity securities    
Held-to-maturity securities, Fair Value $ 1,753,342 $ 1,635,913
Yield, Held-to-maturity securities    
Held-to-maturity securities, Yield 3.15% 0.00%
Investments $ 3,004,083 $ 2,903,443
Gross unrealized gains on investments 19,009 20,972
Gross unrealized losses on investments (23,895) (22,293)
Total financial assets $ 2,999,197 $ 2,902,122
Yield on investments 3.08% 3.02%
U.S. government and agency securities    
Amortized Cost, Available-for-sale securities    
Within 1 year $ 3,300 $ 9,300
1 to 5 years 2,123 5,688
5 to 10 years 68,326 69,108
Over 10 years 122,171 127,936
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains, Within 1 year 70 146
Gross unrealized gains, 1 to 5 years 0 2
Gross unrealized gains, 5 to 10 years 0 0
Gross unrealized gains, Over 10 years 215 353
Gross unrealized losses, Within 1 year 0 0
Gross unrealized losses, 1 to 5 years 0 0
Gross unrealized losses, 5 to 10 years (1,174) (1,238)
Gross unrealized losses, Over 10 years (162) (218)
Fair Value, Available-for-sale securities    
Within 1 year 3,370 9,446
1 to 5 years 2,123 5,690
5 to 10 years 67,152 67,870
Over 10 years $ 122,224 $ 128,071
Yield, Available-for-sale securities    
Within 1 year 9.80% 10.38%
1 to 5 years 1.61% 1.51%
5 to 10 years 2.03% 1.93%
Over 10 years 2.09% 1.92%
Equity securities    
Amortized Cost, Available-for-sale securities    
1 to 5 years $ 500 $ 500
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains, 1 to 5 years 3 22
Gross unrealized losses, 1 to 5 years 0 0
Fair Value, Available-for-sale securities    
1 to 5 years $ 503 $ 522
Yield, Available-for-sale securities    
1 to 5 years 1.80% 1.80%
Corporate debt securities    
Amortized Cost, Available-for-sale securities    
Within 1 year   $ 0
1 to 5 years $ 63,657 63,622
5 to 10 years 119,962 119,960
Over 10 years 0 0
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains, Within 1 year   0
Gross unrealized gains, 1 to 5 years 1,936 2,083
Gross unrealized gains, 5 to 10 years 213 210
Gross unrealized gains, Over 10 years 0 0
Gross unrealized losses, Within 1 year   0
Gross unrealized losses, 1 to 5 years 0 0
Gross unrealized losses, 5 to 10 years (1,352) (577)
Gross unrealized losses, Over 10 years 0 0
Fair Value, Available-for-sale securities    
Within 1 year   0
1 to 5 years 65,593 65,705
5 to 10 years 118,823 119,593
Over 10 years $ 0 $ 0
Yield, Available-for-sale securities    
Within 1 year   0.00%
1 to 5 years 3.14% 2.96%
5 to 10 years 2.66% 2.62%
Over 10 years 0.00% 0.00%
Municipal bonds due    
Amortized Cost, Available-for-sale securities    
Within 1 year $ 2,352 $ 2,344
1 to 5 years 1,374 1,367
5 to 10 years   0
Over 10 years 20,338 20,343
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains, Within 1 year 0 10
Gross unrealized gains, 1 to 5 years 8 55
Gross unrealized gains, 5 to 10 years   0
Gross unrealized gains, Over 10 years 2,144 2,505
Gross unrealized losses, Within 1 year (4) 0
Gross unrealized losses, 1 to 5 years 0 0
Gross unrealized losses, 5 to 10 years   0
Gross unrealized losses, Over 10 years 0 0
Fair Value, Available-for-sale securities    
Within 1 year 2,348 2,354
1 to 5 years 1,382 1,422
5 to 10 years   0
Over 10 years $ 22,482 $ 22,848
Yield, Available-for-sale securities    
Within 1 year 1.23% 1.23%
1 to 5 years 2.05% 2.05%
5 to 10 years   0.00%
Over 10 years 6.45% 6.45%
Agency pass-through certificates    
Amortized Cost, Available-for-sale securities    
Without single maturity date $ 825,744 $ 828,069
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains, Without single maturity date 8,181 8,402
Gross unrealized losses, Without single maturity date (2,436) (2,174)
Fair Value, Available-for-sale securities    
Without single maturity date $ 831,489 $ 834,297
Yield, Available-for-sale securities    
Without single maturity date 3.11% 2.96%
Amortized Cost, Held-to-maturity securities    
Held-to-maturity securities, at amortized cost $ 1,750,886 $ 1,646,856
Gross Unrealized Gains / Losses, Held-to-maturity securities    
Held-to-maturity securities, Gross unrealized gains 6,218 7,143
Held-to-maturity securities ,Gross unrealized losses (18,767) (18,086)
Fair Value, Held-to-maturity securities    
Held-to-maturity securities, Fair Value $ 1,738,337 $ 1,635,913
Yield, Held-to-maturity securities    
Held-to-maturity securities, Yield 3.16% 3.14%
Commercial MBS    
Amortized Cost, Available-for-sale securities    
Without single maturity date $ 8,350 $ 8,350
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains, Without single maturity date 16 41
Gross unrealized losses, Without single maturity date 0 0
Fair Value, Available-for-sale securities    
Without single maturity date $ 8,366 $ 8,391
Yield, Available-for-sale securities    
Without single maturity date 3.55% 3.31%
Amortized Cost, Held-to-maturity securities    
Held-to-maturity securities, at amortized cost $ 15,000  
Gross Unrealized Gains / Losses, Held-to-maturity securities    
Held-to-maturity securities, Gross unrealized gains 5  
Held-to-maturity securities ,Gross unrealized losses 0  
Fair Value, Held-to-maturity securities    
Held-to-maturity securities, Fair Value $ 15,005  
Yield, Held-to-maturity securities    
Held-to-maturity securities, Yield 2.23%