Fair Value of Financial Instruments - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments, at fair value	$ 5,898	$ 6,600
Financial Assets	4,993,582	5,169,996
Significant Other Observable Inputs (Level 2) | Carrying Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets	1,039,144	1,097,801
Financial Liabilities	262,500	262,500
Significant Other Observable Inputs (Level 2) | Carrying Value | Senior Notes | Senior Notes - MHLA – 6.625%
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities	$ 110,000	$ 110,000
Stated interest rate	6.625%	6.625%
Significant Other Observable Inputs (Level 2) | Carrying Value | Senior Notes | Senior Notes - MHNC – 7.75%
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities	$ 152,500	$ 152,500
Stated interest rate	7.75%	7.75%
Significant Other Observable Inputs (Level 2) | Carrying Value | Held-to-maturity securities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets	$ 979,018	$ 1,037,464
Significant Other Observable Inputs (Level 2) | Carrying Value | Held-to-maturity securities | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets	60,126	60,337
Significant Other Observable Inputs (Level 2) | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets	1,026,672	1,125,626
Financial Liabilities	230,277	250,229
Significant Other Observable Inputs (Level 2) | Fair Value | Senior Notes | Senior Notes - MHLA – 6.625%
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities	92,356	101,200
Significant Other Observable Inputs (Level 2) | Fair Value | Senior Notes | Senior Notes - MHNC – 7.75%
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Liabilities	137,921	149,029
Significant Other Observable Inputs (Level 2) | Fair Value | Held-to-maturity securities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets	967,211	1,065,245
Significant Other Observable Inputs (Level 2) | Fair Value | Held-to-maturity securities | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Assets	$ 59,461	$ 60,381