Shareholders' Equity - Share Issuances and Purchases (Details) $ / shares in Units, $ in Thousands				3 Months Ended		6 Months Ended
	Jun. 30, 2017 employee $ / shares shares	Jun. 15, 2017 trustee $ / shares shares	Feb. 10, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) employee $ / shares	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) employee $ / shares	Jun. 30, 2016 USD ($)
Share-based Compensation Arragement by Chare-based Payment Award [Line Items]
Reduction to net income available to common share holders | $				$ 0	$ 0	$ 9,893	$ 0
Series D preferred shareholders'
Share-based Compensation Arragement by Chare-based Payment Award [Line Items]
Shares redeemed (in shares) | shares			11,600,000
Preferred stock, dividend rate, percentage			7.125%			7.125%
Preferred stock, liquidation preference (in dollars per share) | $ / shares			$ 25
Dividends payable at redemption | $			$ 291,435
Reduction to net income available to common share holders | $						$ 9,893
Five trustees | Common Stock
Share-based Compensation Arragement by Chare-based Payment Award [Line Items]
Shares granted in period (in shares) | shares		3,000
Value of grants in period (in dollars per share) | $ / shares		$ 30.12
Number of trustees | trustee		5
Former employees | Common Stock
Share-based Compensation Arragement by Chare-based Payment Award [Line Items]
Shares repurchased (in shares) | shares	499
Share price of shares repurchased (in dollars per share) | $ / shares	$ 29.15			$ 29.15		$ 29.15
Number of employees who sold stock repurchased | employee	2			2		2