Indebtedness (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Subsidiary Debt
Debt Instrument [Line Items]
Face Value	$ 12,517,000
Carrying Value	12,516,000	$ 5,554,000
Repayments in 2019	2,000,000
Repayments in 2020	11,000,000
Repayments thereafter	30
Senior Notes and Other Debt
Debt Instrument [Line Items]
Face Value	1,779,671,000
Carrying Value	1,769,052,000	1,760,595,000
Senior Notes \| Senior Notes due August 15, 2019
Debt Instrument [Line Items]
Face Value	140,651,000
Carrying Value	$ 140,434,000	140,301,000
Interest Rate	6.15%
Senior Notes \| Senior Notes due September 15, 2019
Debt Instrument [Line Items]
Face Value	$ 300,000,000
Carrying Value	$ 299,562,000	299,308,000
Interest Rate	7.375%
Senior Notes \| Senior Notes due September 15, 2020
Debt Instrument [Line Items]
Face Value	$ 300,000,000
Carrying Value	$ 299,083,000	298,747,000
Interest Rate	5.375%
Senior Notes \| Senior Notes due January 1, 2022
Debt Instrument [Line Items]
Face Value	$ 76,503,000
Carrying Value	$ 76,210,000	76,151,000
Interest Rate	8.70%
Senior Notes \| Senior Notes due March 15, 2022
Debt Instrument [Line Items]
Face Value	$ 350,000,000
Carrying Value	$ 348,252,000	347,834,000
Interest Rate	4.625%
Senior Notes \| Senior Notes due February 15, 2037
Debt Instrument [Line Items]
Face Value	$ 250,000,000
Carrying Value	$ 247,896,000	247,786,000
Interest Rate	6.25%
Senior Notes \| Senior Notes due August 1, 2044 [Member]
Debt Instrument [Line Items]
Face Value	$ 350,000,000
Carrying Value	$ 345,099,000	344,914,000
Interest Rate	4.75%
Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Aggregate Indebtedness	$ 290,000,000	281,099,000
Face Value	110,000,000	105,952,000
Carrying Value	$ 728,218,000	340,579,000
Junior Subordinated Debt [Member] \| Five Point Six Two Five Percent Subordinated Debentures [Member]
Debt Instrument [Line Items]
Interest Rate	5.625%
Junior Subordinated Debt [Member] \| Five Point Nine Percent Subordinated Debentures [Domain]
Debt Instrument [Line Items]
Interest Rate	5.90%
Junior Subordinated Debt [Member] \| Five Point Eight Percent Subordinated Debentures [Member] [Domain]
Debt Instrument [Line Items]
Interest Rate	5.75%
Subordinated Debt [Member]
Debt Instrument [Line Items]
Face Value	$ 350,000,000
Subordinated Debt [Member] \| SubordinatedDebenturedueApril30,2053 [Member]
Debt Instrument [Line Items]
Carrying Value	340,838,000
Subordinated Debt [Member] \| SubordinatedDebenturedueMarch1,2056 [Member] [Member]
Debt Instrument [Line Items]
Carrying Value	106,055,000
Subordinated Debt [Member] \| Subordinated Debenture due June1, 2056 [Member]
Debt Instrument [Line Items]
Carrying Value	281,325,000
Subordinated Debentures Subject to Mandatory Redemption [Member]
Debt Instrument [Line Items]
Face Value	$ 750,000,000	$ 727,630,000

Total current liabilities of the broker-dealer, less certain exceptions as defined.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

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Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-SubTopic 210
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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